Equip cards are like enhancements - once they are used they are used, it's not even possible to remove them. Retail had an item that could be used to remove them but that cost more real cash if I remember correctly - doesn't work here anyway. I could probably code it but to be honest, I really wouldn't want to, because cards are the main source of donation income. I already massively reduced that income by allowing you to trade in other cards to get All Stats and making them drop everywhere, and introducing other cards, so allowing you to remove and reuse/trade them just isn't going to happen. It wouldn't hurt the economy, it would completely destroy it, because there would probably be no server to speak of, it would close down due to lack of funds to keep it running.
Your idea about selling things back to an NPC for a fraction of the cost isn't a bad one. I already did this to some extent with Scrub's Tear costumes that you can decompose at Barecalf and get some tears back - I guess it could be done with moolah costumes and possibly even mounts, so you could at least get something back instead of it sitting in your inventory. I will think on that one.