Alchemy

In Halted Stream Mine you'll find the Transmute Mineral Ore spell. Buy or gather a bunch of cheap Iron Ore and transmute it to Silver Ore, and then transmute that to Gold. Turn around and make as many Gold Rings as you can and sell them. Alteration, Smithing, Speech, and profit. :)

Hidden Chests

In Skyrim there are several hidden merchant chests but probably the easiest one to get to is on the east side of Dawnstar near the entrance to Iron-Breaker Mine. The chest here contains all the Khajiit traders' inventory: enchanted gear, gold, weapons... A little bit of everything.

Reset Merchant Gold

Still need to sell more loot but the merchant ran out of gold? No problem. Quick save, smack the merchant with your favorite weapon and immedately load your quicksave. When you load back into the game the merchant will have all their gold back and you can continue your selling spree!