offer or invitation to treat

Offer: an expression of willingness to contract on the specified term without further negotiation, so that it requires only acceptance for a binding agreement to be formed

Invitation to treat: an indication that the invitor is willing to enter into negotiations but is not prepared to be bound immediately

An offer must be communicated, and could be made to a particular person, to a group of people, or to the whole world (Carlill v Carbolic Smoke Ball).

Fact here is not clear whether A intended to be bound without further negotiation or not.
If A's letter is merely an advertisement to be posted in the car club for attraction to interested buyers for further negotiation, this is clear that it is an invitation to treat: Partridge v Crittenden.
If A's letter has included with specific terms, e.g. the price of the car, the condition and detail of the car, etc, with A's own contact detail, when an interested buyer from the car club comes in and "accepts" this letter, A will be bounded as it is a valid offer with acceptance.