Well, there are a couple of things I do with the used decks :
1. Use them as cardistry trainers for block cuts (if they are that sticky, yuck)
2. Make gimmicks with them. Trying out new things with elastic threads, cutting.. basically ruining the cards even more.
3. Recycling the really bad decks.
4. There is a small magic community where I live with a lot of young kids that are crazy skilled. Some of them can't afford to buy cards to practice, so I share a lot with them, trying to support the art as much as I can. That doesn't necessarily go only for used decks, though.
Have a great Sunday all!
Nikolay