Trainer merchants! Pay for training, then sell your loot. At higher levels, buying just one or two skill-ups will give the merchant thousands of gold to trade with. All the merchants at College of Winterhold are trainers, then there's Grelka, Eorlund Greymane, Arcadia, Babette, Balimund and I'm sure there are more. Couple that with the merchant perk for best experience.

I always invest in the Speech tree, so by the time I'm done with it all the merchants I'd sell to have over 2000 gold on hand. The problem, of course, is getting to that point, because the limited gold they start with makes it tedious to sell large amounts of a given type of item. The Merchant perk speeds it up significantly, since I can sell the same stuff to a lot more vendors, but their cash on had remains limited until I can invest. That makes selling valuable items a real pain, since I can't stand eating large losses in the process, so I often end up sitting on a huge pile of really valuable items until I can invest in Lucan Varelius' shop in Riverwood, since that gives him over 10K gold.

I'm thinking of increasing the base gold merchants have, in order to speed up the selling process. Yes, that means I'll raise cash at a ridiculous rate, but that's actually a good thing; I made it possible to train a huge number of times per level and greatly increased the fees, so all that cash disappears in short order when I decide I want to raise a skill. Even without the training mod faster selling would be a good thing, since it would leave more time for adventuring.

To add to my reasons for going into dungeons I'd like to add that I like to kill things. Re-clearing a dungeon is not something I do often, but I have done it just to hang out with some follower messing around with ways to kill stuff. Same argument for MMOs. I enjoy the activity of raiding and that's why I raid. Getting better gear and getting better at raiding just add to my bottom line which is I like to spend my time raiding. But I know to a lot of people loot is the pay off, and they feel like they've wasted their time if they don't get any actual reward.

Have you tried the Golden Touch perk in the Lockpicking tree? I've never played with this - to me perk points are the more precious than gold - but with this perk you will loot more gold from dungeons, so you get richer without ever having to sell anything.

Edit: I forgot to mention that I do not believe there are any Vanilla 'Uber Merchants' in Skyrim like Creeper, The Ashlander guy and the Mudcrab Merchant in MW, because the vendor gold being rare seems to be the incentive for spending quite a few perk points, mostly in the Speech tree. But the logic hasn't stopped me from keeping my eye out.

The only 'uber' merchant is Lucan Valerius, however you have to have the Investor perk from Speech and drop $500 on his shop before he'll have the 10K+ bankroll. Somewhat ironically, by the time you 'unlock' him you probably have so much money that it no longer matters if you can sell all that valuable crap or not.

I'm a pack rat and I loot everything, so for me the wight/ sell is always a problem.
I first get the steed stone, then buy a house... buy them all if I can invest time and money at that point in the game (leave extra loot in house for next time I run through town and merchants have restocked their gold and other goodies).
Make carry weight potions and carry weight enchantments. Lvl up speech and get the merchant perk(sell all to all).
When selling I sell first to merchants that have nothing I want, or very little of it. Then move on to those that have things like soul gems, ingots, alchemy ingredients, potions, and buy it all up trading back the stuff I wish to sell afterwards.
Training with merchants helps, but for me it takes the fun out of the game.
Do I need all that money at lvl 20 or 30? Yes I do. Consider all the things I just said I buy up, consider the price on those. The only people that have no value for money are the ones that do not buy anything, and leave alchemy, smith and enchantment trees at a slow crawl, not even mentioning the speech tree.

Sell. Sleep a day. Repeat?

You don't need to buy a single thing to very quickly level up alchemy, smithing, and enchanting.
Even without killing giants, you can make quite a few potions with very good value (pretty much anything that synergizes with bearclaws and hanging moss), and of course making paralysis potions both boosts you skill and makes killing giants (and bears) easy.
The items for crafting iron daggers are easy to acquire by hunting and mining.
Soul gems are quite amply available in Dwemer ruins (especially Blackreach), you just need to fill them. Have fun enchanting those daggers you just made.

Doing these things not only costs no money, but produces items that you can sell for a very good price, if you wish to do so (and in doing so, raise speech craft).

I fully admit to abusing Trainer Merchants. I like Grelka and Balimund in Riften for armor/weapons (Grelka takes misc items too) and Arcadia in Whiterun for potions. Revyn Sadri at Sadri's Used Wares (speech trainer) in Windhelm looks like a good choice too.

For the vanilla game, if you don't wish to invest perks in the Speech tree, you can increase a given city's available merchant gold by doing bounty quests. In my experience, turning in a normal (bandit) bounty will increase all shops' available gold by 10, and turning in a dragon bounty will increase by 100 gold. Happy hunting!

I remember someone said the giants toe has carry weight, but I never said I go kill giants. In all my time playing since November... I've only killed two.. perhaps three.

On leveling Alchemy... there are lots of ingredients that you don't easily find out in the wild, and any decent alchemist will want to know all you can get out of each. Not everyone has a lot of time on their hands to put into the game, you choose time or septims, I chose septims.

On killing things to get ingredients. Some creatures are easy enough to find... others are a pain and/or can only be found in small numbers. That's when cash comes into play, or you could spend month's trying to round up the necessary amount by killing things.

On crafting daggers. Where do you get all that iron? Do you know how much it takes? Don't tell me you bought it... anyway... even if you have a good enough memory to remember all the iron mines so you can loop through them, you are still bumping onto opposition... which means loot... which means more to sell... which means I got back to my original problem being a pack rat.
I was aware about the Dwemer ruins being a good source for smithing and enchanting, but thank you. It's just not enough... and while clearing those... did I mention I'm a pack rat? It takes me ingame months to clear those ruins. Lots of dwemer smithing done... not too much enchanting.
...and enchanted daggers cost more coin... so beside offering me a tedious system to follow... I'll be heaving even more trouble getting all my items sold.

I bought most of my iron from money I made chopping wood. Halted Stream Camp north of Whiterun has a decent amount of iron ore, but not enough for as fast as I like to level Smithing so I buy iron and leather straps. There are also a few mines around Riverwood I believe. It takes quite a bit to get to 100, but I grind it out because I don't like doing many of the quests until I have really good gear.

That was the point I was trying to make. I was told that these skills can go up fast without using money, but I never could get alchemy, smithing, and enchanting to levels I want and at the speed I want without using money. Trading in loot for when the merchant does not have any money works. I sell the loot I don't want and gain perishable items (soul gems, ingredients and ingots for upgrading armor before it's sold) that helps me increase my skills. I don't have to lug around all the loot I could not sell and I didn't have to toss it or leave it.

Yeah, I always prefer farming the materials (feel more of a sense of accomplishment). I slowly farmed all the materials for Alchemy and Enchanting, but I bought a lot of the materials for Smithing 100. I paid a Trainer a few times in Alchemy on my chick and Enchanting on my dude, but never bought any of those mats.

I end up buying arrows, no weight, then selling off my stuff. a few merchants also train, train first then sell.

So why ARE items valued at more than they are...valued. Are they telling me how much I'll pay from a vendor? Why do I need to know that when I already have one in my possession? I need to know what it's worth if I want to sell it......

Yeah, that annoys me too. I think it displays the 'retail' price and they buy it from us for the 'wholesale' price.

What things are worth if you want to sell them: approximately 1/3 of the value shown in your inventory list. This will vary a bit (though not gobs) based on your speech skill and anything with +barter you're wearing. Just like RL - markup 300%....

I've got a few pieces of gear that allow me to receive 82% of the 'retail' value. Still need that Fortify Speech enchant before I enchant my own stuff - maybe then I'll get more than 100% of the 'retail' value.

That would be nice, but there are caps in place that prevent you from selling an item for more than it's worth or buying for less. Those on a PC can change them so that you actually can sell/buy for more/less, but care should be taken when doing so as even small increases/decreases in their values can cause large price shifts in valuable items. As for the values themselves (they're applied as multipliers):

fBarterBuyMin == 1.05fBarterSellMax == 1.00

The latter one means that you can, with sufficient Barter buffs, sell items for face value, assuming the merchant can actually cover it.

I'm on PS3 so I can't manipulate with console commands, but it'll be interesting to see what prices I can get when I actually enchant items with Fortify Barter and Fortify Speech.

But this is why I don't need money from loot. I maxed out all 3 crafting skills early on and was able to make myself weapons, armor, poisons and all the potions that I needed. Anyone who has ever been a follower of mine has also gotten a bow, sword and set of double enchanted armor with their name on it. Even if I never sold a thing I made, I still have pretty much everything I need because I can make it myself. I like this feeling of skilling up and getting completely prepared before i go out adventuring, which is what I enjoy most.

I made my sword before I ever quested much, so now we have a long history together, which I enjoy more than getting to end game and then 'finding' some cool sword.

The hints for getting better prices from merchants don't really address the OP's problem which is that merchants never have enough cash to buy off all the stuff he/she's hauled out of dungeons. Without spending precious perk points in the Speech tree, the Vanilla game is going to demand that you just dump it on the ground and wait until their cash is refreshed.

I just wouldn't spend perks in Speech - or The Golden Touch perk in the Lockpicking tree - simply because I want those finite perk points in combat related areas, and there is plenty of gold to be had in the game without spending them.

For RPing a revisit to a dungeon you've already explored, why not try taking jobs to kill x at y and just enjoy clearing through whatever dungeon you've been given without having to micro-manage looting? In this situation I still check corpses and containers in case there's anything unusual and I also pocket any coin at the same time.