Pay for the xp cost of new magic items with xp cost in other magic items that you may gain over the course of your game. The game says you can only use half the resources of an old item to create a new one; instead of using half the materials and half the xp, they can just use the entire xp-half and discard the salvageable materials.
Afterward, you can cover up for any minor shortages with XP from the players themselves, which shouldn't hinder them too much. That way, they get the magic items they want, but lose the magic items they already have (this also helps deal with the unwanted magic item issue better than 'HEY, TOKEN WIZARD SHOP IN FRONT OF THE DUNGEON, BUY THIS PLZ').
Leave the XP-only system in for players who are still willing to sacrifice their XP for an impressive magic item; allow the exchange system to supplement this, and cut out some of the creation time due to the XP cost already being available.
May or may not require some house-ruling, but who cares. It's smarter and fairer this way, and nobody actually loses anything, they just have more options to choose from.