If you need gold and have a rogue this works well. After you have sold many many things toa vendor you can sneak behind them and if strong enough backstab them in one hit. No guards will be alerted and no loss in rep. Anything you sold will drop. Now go sell it to someone else. Buy everything in the land!
You'll soon run out of NPCs with enough money to buy your stuff though. I only found about 10-15 NPCs with good amounts of cash in the whole game! The three sitting in the pub in Silverglen are the best!
I killed the sailor captain at the harbour in cyseal and the witch in the inn. Both of those after selling to them every level up, netted me about 50000 gold in the wares when i sold them back.
gone for good, every level up their inventories reset exept for what you've sold them. I did this at level 13 by then i had sold a lot to those 2 vendors.