My Mercantile is 61, a long way away from 75. After getting loads of stuff off of FCOM and loading it onto CM Partner Azazel, I always go to Rohssan in A Fighting Chance. She has 1200 gold, is there anyone better than her? Who really has the best prices in all of Tamriel? The reason I need money is... training, training, training, and enchantments, enchantments, enchantments!

1200 gold is the maximum gold available from any of the base game's regular merchants. Fathis Ules, a fence in the Elven Gardens, has 1500. A few merchants added by DLCs have 2000: Aurelinwae from Wizard's Tower, Nilphas Omellian from Fighter's Stronghold and Rowley Eardwulf from Vile Lair. There may be a few more fences I have forgotten. Glargg will know if there are.

Here's a link to the merchants that includes how much gold each has: http://www.uesp.net/wiki/Oblivion:Merchants#Merchants

As you can see, the merchants with more gold also have fairly high mercantile skill making them a bit harder to bargain with.

Because of the max amounts you can sell one item for, you might consider ignoring the value to the extent that it exceeds that amount. For example, a dagger of doom worth 1800 that weighs 5 is a better deal to lug out of a dungeon than the mighty warhammer of smiting valued at 4000 that weighs 70.

Glargg is still learning new stuff every day. I didn't know about the higher DLC merchants.

I agree with Acadian about the weight-to-value ratio. It's not practical (or realistic, in my opinion) to haul large amounts of looted armor out of caves and drag it all back to a merchant miles away. The most valuable stuff, from a weight-to-value standpoint, isn't the heavy Daedric junk. It's arrows, rare ingredients, rare books, scrolls, etc. Enchanted light armor and weapons are not bad to carry, but my characters mostly just leave the heavy stuff (unless it's something they'll use.)

i would love if there was a mudcrab merchant or a Deadroth Merchant with 20000 gold

The amount merchants are willing to pay depends on their Mercantile skill - the lower their skill, the more they'll pay, at least up to their maximum.

The most generous vendor - and not coincidentally the one with the lowest mercantile skill - is Sinderion in Skingrad, but he only has 400 septims and only buys and sells potions and ingredients and such.

Rohssan is actually not at all generous, and that's true of most of the vendors with the most money - they also have the highest Mercantile skill and thus pay the lowest prices. The merchant with the most money in the vanilla game is the fence Fathis Ules, and he's also a Mercantile master and extremely tight-fisted. Not to mention you have to be in the TG to deal with him.

The most generous vendor in the Market District is Rindir, though again, he doesn't have much money (only 600 septims). He does buy a pretty broad range of stuff though, and is far and away the merchant I deal with most often.

The best merchant in the vanilla game is Varel Morvayn at Morvayn's Peacemakers in Anvil. He has the best combination of high money and low mercantile skill, so he pays good prices and can afford to pay more (1200 septims).

Another notably good one is Daenlin at The Archer's Paradox in Bravil. He has fairly poor Mercantile skills and 1000 septims.

Most of the Mages Guild merchants are poorly skilled and thus generous, but they don't have much money and they usually only buy potions and ingredients and alchemy equipment.

I was kind of sad to see them go myself. I thought they were fun. But they really divided players back in the day. Some loved them and a very vocal portion of players really hated them. These players thought creature merchants were "cheats" and including them was just one more example of how far Bethesda had fallen into mediocrity when they made Morrowind. Creature merchants were not just "cheats" to these players, they were ridiculous cheats (a Mudcrab?) Bethesda took a lot of heat for those. And frankly I was not surprised that they were not in Oblivion.

But, one might postulate, if one felt inclined to do so, that they introduced an equivalent "cheat" in Oblivion that makes getting rich as easy as using a creature merchant. And that is: merchant gold never ever fluctuates. We can sell 30 Glass cuirasses to Rohssan in a single bartering session and her gold will always be 1200. To my way of thinking, this is, to borrow a phrase from Frank Zappa, a cheepnis worse than creature merchants (since it cannot be avoided). So I guess it all evens out in the end...

My mercantile skill never gets up high enough to make the traders deal in all items, and I am too lazy to visit multiple stores. Places like Anvil dock and Jensine at Market District are where you'll normally see me.

Perhaps one day I will stop being lazy and start getting some good deals. Lol. Oh, and Bravil at night, always a good place to sell in a pinch (very low amount, though).

When collecting items from dungeons I do look at the gold/weight value. Unless it is 3 or 4 to 1 I will generally leave it there.