oh yeah sure. The idea is, as you said, a listing fee. How it is implemented doesn't matter. As long as changing the price cost something, the idea is here.

But I think only one "tax" is suffisent. So I would say, fix listing fee of 5% is a good deal.

More if you put a biiig fee (like 20+%) we'll see some chat market emerging to prevent to pay this big fee. I know this will never append because it goes against the "mainstreaming, less player interaction possible" way that SE is currently following. But this will prevent botting like crazy.