An advertised price is NOT legally an offer, although it is very often stated as such which leads to confusion.
The advertised price is simply an INVITATION for offers.
In legal terms, it is not until the buyer says they will pay that the offer is made. So, it is the BUYER that actually makes the offer.
Example:
Advertisement is an invitation to purchase for £100.
Buyer offers £90. (This is the begining of a contract).
Sellar accepts, The contract is formed, so long as there is:
Capacity. (Buyer is of age and mental ability).
Consideration. (payment in cash or kind).
Most sellers will accept an offer for the advertised price, even if the advert is a mistake. But they do not have to.
This is from a legal study sight for students. (Even this is incorrect, sadly).
This is the correct interpretation of contract law in this respect. (It also agrees with what I did at college).
John