There's another dynamic here - something you could do in Morrowind and returned in Skyrim (but was absent in Oblivion):
In Oblivion, the "Invest" perk and "Master Trader" actually meant something, sort of, as vendors would only ever has as much money as they had. If they had 800 septims, you couldn't increase their money.
In Morrowind and Skyrim, if you know who you are going to buy from, and let's say you want to buy 3000 gold worth of good and/or training, well, you make your purchase (assuming you have 3000 gold), and now the shops cash is increased by 3000GP, to say, 3800 total. Now, you have 3500 or 3700GP worth of items to sell, you can sell them and get your money back. So, that's basically what I do. I took a couple levels of Speech's basic perk (which increase your sell price and decrease your buy price by like 10% each level), so that I didn't get ripped off so bad when buying and selling, and sometimes use fortify haggle potions, and I usually go find a beggar to get the 1gp Gift of Charity, so I get decent prices, then I just do what I outlined above.
I could see how investing might still be somewhat attractive, but as long as you have stuff to sell when you have stuff to buy, investing becomes less necessary.
EDIT:
On a side note, I'm sure everyone is buying everything from a store that they might want, before selling an expensive item worth more than the merchant has. I usually buy up all ingredients and soul gems.
Guess someone did mention it before, and I just missed it on my first read through.