What merchant has the most gold?

Who can I sell to that has boatloads of cash?

Ideally they'll be easily accessible by fast travel and will buy the most different types of items, but a rich type-specific merchant will work too.

If you complete the restore the Thieves Guild to its former glory quests, all the fences will have 4000 gold to barter with. You can further boost this via investing and the speech perk for a total of 5500 per fence.

There is a guy at the Riverwood trader who has 10000 gold once you invest with him. I don't know if it's a glitch. This was on the Xbox before the first update so it could be fixed.

Selling things in Riften is good, especially if you are part of the Thieves Guild. Then you'll have 5 merchants to sell things to. 3 in the market, 1 smith and Tonilia in the Ragged Flagon.

I found this because I had the same question, and after reading the available answers, I can offer a trick that might still help.

Sell as much as you can/want to a merchant, then SAVE the game. Then, kill that merchant. Once they're dead, simply reload the save you created before killing them. Their inventory and gold will be restocked. Just keep doing this until you've sold all you want to sell.

I know this doesn't answer the question specifically, but I still thought it might be helpful.