According to Law Teacher, the difference between an offer and an invitation to treat is that an invitation to treat occurs when a client invites contractors to make him an offer, while an offer occurs when the client offers the job to one contractor without advertising the job or allowing other contractors to submit tenders. Making an invitation to treat instead of an offer protects a client.
An offer usually takes place when one party indicates that he is ready to contract with another party or parties on a certain agreed set of terms. In case of a contract dispute in court, the judge tries to establish whether one party agreed to the terms set out in the agreement, and whether there was a breach of the contract. The courts usually try to distinguish an offer from an invitation to treat by objectively asking whether the party intended to be bound by his statements. An example of when a court may infer that the dispute involves an invitation to treat instead of an offer is when one party was merely hoping to start negotiations. A classic example of an invitation to treat is when there is a low-priced product on display in a shop, but the shopkeeper is not obligated to sell the item.