I am broke,but I have a ton of stuff to sell.Like 23 flawless rubies.I I cant sell more than 1 or 2 to any merchant!Where can sell 20 K worth of loot?

the merchants with the most gold are the ones that show up in the ragged flagon after the thieves guild special jobs

Sell what you can to a merchant, save and then kill the merchant. Reload and they'll have their inventory/gold restored, allowing you to sell again.