Such an offer (bid), when accepted by the fall of hammer or in some other customary way, will result in a (binding) contract.
A contract is a legally binding voluntary agreement formed when one person makes an offer, and the other accepts it.
Such pre-contractual representations may include "invitations to treat", "requests for information" or "statements of intention".
In certain circumstances called unilateral contracts, an advertisement can be an offer; as in Carlill v Carbolic Smoke Ball Company [1893] 1 QB 256, where it was held that the defendants, who advertised that they would pay £100 to anyone who sniffed a smoke ball in the prescribed manner and yet caught influenza, were contractually obliged to pay £100 to whoever accepted it by performing the required acts.
The shop owner is thus not obliged to sell the goods, even if signage such as "special offer" accompanies the display.
If a shop mistakenly displays an item for sale at a very low price it is not obliged to sell it for that amount.