Merchant's Cursed Item

''But beware the mysterious effects-- ah, you'll just end up buying it anyway. Here, just take it.''

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 jackass customer buys the thing anyway without regards to what the merchant told them about it.

The main thing about the item is if it's not supposed to be bought, then why is it on the market where any moron could just pick it up?