Just make decarding a zeny sink or something. Like only make certain most popular cards be able to be decarded from a drop down in an npc. You can then choose whether to keep the gear or card. Give the more expensive cards a smaller chance to decard and have it cost like 1 -50m depending on the rarity of the card. Like a maya purple would cost like 50m to decard at say like 25% chance. (dependent on server)
no one would use it on lower end cards as it wouldnt even be worth it
If you're going to remove cards from gear for zeny/eden marks/certs it should be a guarantee.