A pretty simple exploit that lets you get as much cash as you want! It's a pretty simple process too, and much less conspicuous than the Gem Dupe bug.
- Find Roger the Fence in the sewers underneath the Temple District. He's one of the few characters who will buy stolen goods and can be stolen from himself.
- Have someone with Pick Pockets (Haer'Dalis, Yoshimo, Jan or your main) drink a Potion of Perception and two Potions of Master Thievery. Ribald sells these at the Adventurer's Mart and Roger has a couple as well. This should push your thief's Pickpocket skill up to well over 100%, which is just what we need.
- Now pick one or two expensive pieces of equipment you own (the higher the price, the better) and sell them to Roger. Then go into the Steal menu and immediately swipe them back.
- Repeat as much as you wish until you're swimming in cash.
Just keep in mind that you still have a small chance to get caught with each theft, so you may want to exit the shop and quick-save every few cycles just in case something goes wrong.
Oh, and before anyone asks:
- The ring I equipped Yoshimo with at the start is the Ring of Human Influence, which boosts Charisma to 18. This helps him get slightly better prices, but is by no means necessary.
- The weapons I'm repeatedly buying and selling in the video (Lolth's Cruel Sting and Charged Short Sword of Wounding +5) are not in the base game; they're introduced by WeiDU's Underrepresented Items mod.
This is done on the original version of BG2. It apparently still works on the Enhanced Edition, but I have not yet tested this myself.