What I would really like to see was something like this for every player, maybe besides the current market:
You put in all cards you want to sell, you can either do this for something or just make it "await offer".
When you think somebody has a good offer for one of the things you offer, you can accept it.
If everybody has this people will click through these "shops" (maybe accessible via player profile), can search for shops which have a certain card available and when the game grows bigger maybe have an option "only show trades that you can accept right away".
I hope you got what I meant
The normal thing could then be limited to 3 posts a day, people will make cheap offers there, linking to their shops.
Also maybe ebay style auctons? Need bigger community for that tho.