I have read several places that Grelka in Riften gets over 3300 gold after joining the Thieves Guild. I have joined the Thieves Guild and even completed the Nightingale quests, but she still only has 750 gold.

Anyone know what I might have done wrong?

Not grelka, tonilia the fence in the ratway. When you do a few thieves side quests, stealing a gem from a city etc, you will eventually get a larger quest for a specific person in that city. Once you have done all 4 city quests tonilia will have 4000 gold.

unless you lost your thieves guild armor before the quest where you are supposed to turn a piece into her. then she'll have 0 forever

If looking for an exploit, sell all your junk to any trader, save the game, kill the trader, load back the save, voila! he/she has 750g again

OK! But are you sure that not Grelka has it too?

"Great merchant to sell to: Grelka appears in Riften after joining the thieves guild. She has over 3300 gold and will buy everything you want to sell."
http://www.gamingreality.com/2011/11/skyrim-tips-and-hints.html

I hope somebody can help me

Simpler then that,
If you get the speech perk to give 500 gold to a trader. The merchant in Riverwood will have 10,000 gold for selling.

Most likely the site is wrong. Guildmaster and she is a normal merchant for me.

I found out if you do jobs for Guild Grelka's (Not Tolinia's ) gold is increasing too. I did about 30+ jobs for them and Grelka's gold increased to 1500.

Tolinia gold only goes up when you do Special Jobs.

If you want training in Light Armour as well, Grelka takes the payment for that and pools it with her trading money, unlike previous Beth games where it was separate IIRC. But the money is gone after a period of time, so you need to use that window to effectively trade expensive items for training. Of course if you prefer Heavy then you're out of luck...

Hm, you sure that wasn't speech perks? I wouldn't advice getting speech perks, you can get enough money without them.

Not sure about grelkas gold going up, I wouldn't trust the website. Tonilia's gold will go up though when you do the special quests.

I have now done about 10-12 extra jobs for Vex and Delvin (numbers jobs, shill jobs etc.) in different cities. But how many jobs must I take before Tonilia gets 4000 gold? Is there a way to see how many jobs I got left?

If you do bounties for the jarls/holds, all the merchants in the city get a moderate increase to their gold. iirc, it adds 10 gold for each normal bounty, and 100 gold for dragon bounties. At least that's how it seemed to work when I did a small test, in Riften.

You do a certain member of jobs (5 I think) for each city, then you get a special city quest. Each city quest you do her gold increases up to the maximum of 4000.

I think that was a bug they fixed in one of the patches, I could be wrong tough.

I don't think it's a bug, but I don't know how it works. I was playing yesterday and the trader in RIverwood had 6,000, but this was a bit after I took some Speech perks. I never invested the 500 in his business though. It just seems after you take a couple Speech perks, the merchants gold just jumps up after a while.

And for the record, I've done complete playthroughs (level 81, all skills 100) without Speech perks, and it took soooooo long selling loot from dungeons. It's worth the perks just to save some time selling loot. I got the 100,00 gold achievement on a character without the perks, took for-freakin-ever.

It's still active; my current character got Investor last night and gave Lucan Varelius 500g, and he now has 10,500-12,500 or so gold on hand each visit.

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I will move this now.