Bait and switch is a fraudulent practice in which a seller advertises a special deal but makes only a very small number of the items available for purchase.
When a customer tries, unsuccessfully, to buy the item that was offered as bait, the seller tries to convince them to buy an alternative product. Unscrupulous online sellers and bricks and mortar retailers both practice bait and switch. The practice is also used in other contexts. For example, a mortgage agent might advertise the very lowest available rate -- for which few consumers qualify.
Another type of bait and switch is the practice of advertising something at a low price but without disclosing any associated costs that will add to the final total. Airline seat sales are a common example: the posted sales price often omits taxes and surcharges, which can mean that the actual cost is several times the amount of the one advertised.
SophosLabs explains one bait and switch campaign: