Something which would help, but not really solve the problem is to deduce the tax when putting an item on sale and not when it is sold. Let's say you want to sell an item 100k and the tax is at 5%. When you put the item on sale, you pay 5k. And when it is sold you get 100k. If you change the price, you pay the 5% again on the new price. This will prevent undercutting a little bit because, well, you loose money each time you do it.