The Jeweler in Hommlet is one of the only traders that puts the items you sell into his personal inventory--so you can pickpocket them back and resell them over and over.
Once you have emptied his inventory, sell him the most valuable gem you have. Don't sell him anything else--that will just fill up his inventory with stuff you don't want to steal back. There are 6 (or more) jacinths in the Temple (worth about 30K total) that work well for this scheme. Each time you pick his pocket--bingo--you have your jacinths ready to sell back to him.
You should be able to max out your gold very quickly, assuming you have high ranks in appraise and slight of hand. You can help things along by casting heroism, cats grace and fox's cunning on your rogue, and equiping him with gloves of dexterity and the circlet of intellect from the Earth Node.