Bait-and-switch

Bait-and-switch is a trick that sometimes dishonest businesses will use to get people to buy something they don't really want. It works by offering something very attractive—the 'bait'—but when customers go to buy it, the shopkeeper 'switches' the offer, and tries to persuade the customer to buy something more expensive or something they don't want at all. For example, if a shop advertises a big sale on TVs, but when you get there, the shopkeeper tells you that all the cheap ones have already gone and you should buy a more expensive model—that's bait-and-switch.