Merchant's Cursed Item

...but beware the mysterious effects — ah, you'll just end up buying it anyway.

An interesting trinket with magical powers, usually harmful, that is sold by a (crazed) merchant. He gives the customer a brief description of the item, then warns them not to buy said item due to its evil powers. Of course, the idiot customer buys the item anyway without regards to what the merchant told them about it. This will unavoidably lead to the customer getting hurt later by this item, as they deserve.

Real Logic likes to ask a question about the Merchant's Cursed Item: "If no one is supposed to buy it, then why is it on the market where any moron could just pick it up?"