I know the guy in Anvil is good for weapon/armor but who pays the top dollar (or Septims) for clothes/magical rings? Thanks.

Hamlof Red-Tooth, in the jewellery shop in the Imperial City, is OK even though he doesn't pay the best price, because he has no vendor chest so anything you sell him will be on him, meaning you can pickpocket your stuff back off him. For the less evilly minded, he can be used as a loan of sorts, because he never resets so you can sell him something for the quick cash then buy it back from him at any time. He'll also wear stuff he buys from you.

When you reach level 50 with mercantile, you can sell anything to any vendor.

Also when you become Expert in Mercantile you can invest in shops, giving them 500 Septims extra permanently. So if you could only sell something for 1000 gold before you can then sell it for 1500

The Wiki has a good article for just this sort of question. Here's the link http://www.uesp.net/wiki/Oblivion:Merchants

Cheers