- Talk to Khorailla.
- Choose "Do you have anything to trade?"
- Let's say you want to purchase Cooked Perched, and you see she has 6 of them.
- Buy all 6 Cooked Perched.
- Quit the shop.
- Talk to Khorailla again.
- Now choose "Please sell me some of your famous Cheddar cheese."
- Now you will see another 6 Cooked Perched available for you to purchase.
I haven't tried with other merchants. Actually, I don't know if there is any other merchant with similar setup. However, If there is, I am assuming it may have the same exploit.