I know there's been a lot of talk about adding posting fees or sales tax to the exchange. I have been thinking, (this may have been mentioned before so don't flame me if it has) why not just add a return fee? If the item doesn't sell and has to be returned to the seller, then charge a percentage or something. That way, instead of punishing those of us that post items at competitive prices, thereby and limiting the various items available in the exchange, it will make people think twice about trying to sell that vanilla Phaser Beam Array Mk IV for 400,000 energy credits.
If this hasn't been discussed, I implore you to do so. There is really no better way to improve the economy.

CIVILIAN AGENT: Sir, ...I'm sorry, but your voice is carrying. I don't think you want to be discussing this subject in public.

Section-31: I'll discuss what I like, and who the hell are you?

CIVILIAN AGENT: Could I offer you a ride home, Section-31?

Sectio-31: Where's the logic in offering me a ride home, you idiot! If I wanted a ride home, would I be trying to get them to add return fees?

ALIEN: Aarrgh!

CIVILIAN AGENT: Federation Security, sir.

(Section-31 tries the Vulcan nerve pinch on the agent unsuccessfully)
CIVILIAN AGENT: You're going to get a nice, long rest, Section.