About Contract - Please Advise, Thanks

1, The offer was made by Mary to Sam. Sam cannot extend the offer to James unless Mary had authorized him to act as her agent. Mary had the property to offer for sale. But she did not offer to James. Therefore, there is no binding between Mary and James until Mary answered the message from James.
2. The offer was not made by Mary directly to James who got the information of sale offer from Sam. James submitted his offer to buy to Mary. Although Mary answered the message few days later after James left his message on the phone, she accepted Jame's offer to buy. The deal was binding unless she did not accept James' offer to buy.
Then Mary did not conclude the sale with Sam. It was the grey part of the contract. Noramlly an offer to sale and an offer to buy states clearly the terms of the sale, including, the designation, the validity of the offer, price, payment terms etc. Mary made her offer verbally and had omiited most of the terms of the offer. Strictly speaking, the offer to sale should have been limited to accept at the time of offer. Mary was free to offer to others because Sam did not accept the offer at the spot. Therefore, there was no breach of contract (in fact there is no contract at all).
3. The situation of Sam was very simple. He just informed James that Mary had a mobile to offer to sale. He gained or lost nothing. He had no contract with Mary or with James. However, as explained above, James was in a different situation. He wrote a message to Mary to offer to buy and Mary returned a message to James to accept his offer to buy. It was a contractual deal already. What left behind was the negotiation for other terms of the contract.
4. Once Mary sent the acceptance message to James, she could revoke the contract unless she really wanted to do so. James had the right to sue her for breach of the contract.