Ha! That was the first idea that sprang to my mind _if_ you feel a need to actually have magic shops in your campaign.All magic items are cursed until paid for. Sorta like those tags you put on clothing that set off the alarms when you walk out? Yeah same deal.
I vastly prefer that all magic items (excepting the most common types of potions) are custom-created to order. Typically, they also only work for the customer or the person that is intended to use them. This has the interesting implication that you'll never find usable magic items lying around as treasure and the pc's opponents can be decked out in magical armaments without the GM having to be afraid of them falling into the players' hands.
But what if you want the pc's to find cool magic items from time to time?
The solution are 'Legendary Items', an idea that afaik originates in the Earthdawn RPG: Items that slowly reveal more of their true potential as the newly bonded owner gets more powerful, investigates the item's history and completes quests that replicate the deeds that originally shaped the item. This is similar to the concept of 'Weapons of Legacy' in D&D, except those required personal sacrifices of some kind (xp, gold, hp, etc.) to unlock more and improved powers.