Blind trades
José Tavares
Set up a system where you could pair user a that needs card x with users that need card y user a has duplicates of.
In order to match equivalent cards this could be done on the schema level.
Like I want to finish base set of the ultra comix collection but I need 3 cards but have duplicates of 6 cards of the same set.
Tophinity
The best suggestions are the ones you didn't know you needed until you read it. And holy hell did you ever scratch an itch I didn't know I had.
Props to you for this idea. You've got my vote.