I like selling stuff in Whiterun, usually. It's very centrally located and since you tend to spend a lot of time there starting out you're ahead of the game in terms of how much the merchants there like you. But really any city with three or four different types of vendors in relatively close proximity works. The real trick isn't to find out
who to sell to, no one has enough gold for that. It's
where do you find as many merchants possible because between them they might have enough gold to buy all of the crap you just looted.
Also consider that if you do a quest for a merchant then I
believe it raises their disposition towards you, which means better than average prices. So it's worth it to deliver that sword that the Blacksmith lady in Whiterun wants you to take to Jarl Balgruuf, for example.