So, simple idea.
Imagine you got a few items you don't or anyone you know does.
Add an option in the actual shop, that you can sell it for something like half the price you originally bought it for.
I know this could bring some weird stuff if you bought something off the market. But hopefully you get the idea.

Has been posted before, with explanations why it wouldn't work

People would put stuff on the market for like 1 tc and then someone would buy it and sell if for half the original price right?