The invitation to treat is created by the person selling the book by making the advert, the person wanting to buy the book will have to make an offer to this person i.e. "I will buy your book do you accept?"... The offeree will then reply and accept or reject the offer, only once the offer is accepted can he post the £5000, there would be no point sending it if they have not accepted. Knowing the address does not simply mean that the person can send the money because there still needs to be offer and acceptance for there to be a binding contract.
- unless stated in the advert something that explicitly says that they will bind to whoever posts the money first and thus creating a unilateral contract by which the bookseller would be the offeror and the person buying the book would be the offeree partaking in a unilateral contract that only needs the offeree to perform an action (the posting of the £5000).