The case concerns about the formation of contract. A contract is the agreement which formed between two opposite parties which is enforceable with courts. The essential factors in a contract include an offer, an acceptance, consideration, the capacity to enter, and intention to be legally bound.  In this case, John entered into negotiation with two parties as to the sales of his antique. Analysis shall be first given as to whether any contract enforceable was formed.
As to the case of Monica, there was no contract formed. The formation of a contract requires a legal agreement, which means that the offer to sell shall be accepted by the offeree. Under this principle, the offer to sell was made by John. However, from the facts given, Monica did not accept the offer as she was given time to think about the agreement.  Thus, no agreement was made. Next is about the contract with Mark. Mark and John formed a memorandum of the sale which contained the terms of the contract.  The memorandum works the same as a contract which means that it can be enforced. However, the enforcement of the memorandum comes with a condition which is that John shall consult with legal advice as to what contractual obligations he has as to Monica. Only under the condition that John did not owe any contractual obligations to Monica can the agreement between Mark be enforced. As is pointed out above, John made an offer to Monica. And a contract would be formed the minute that Monica accepted. Therefore, there would be two consequences which depended on the reactions of John. First, john could just wait for the answer of Monica. If she accepted his offer, then the antique shall be sold to Monica. If she rejected John’s offer, then no contractual obligation shall bound John. Consequently, Mark can get the antique according to their memorandum.
The second option is that John could just revoke his offer. Under most conditions, an offer can be terminated under the following conditions: revocation, rejection, lapse of time, death, and conditions which may terminate the offer.  An offer can be revoked before the offer is accepted and during the time limit which the offer is said to remain open unless consideration is involved. The revocation of an offer shall be communicated to the offeree so that it can take effect. The revocation of an offer takes effect when it is received by the offeree. In Byrne v Van Tienhoven (1880) 5 CPD 344, on the October 1st, the defendant made an offer to the plaintiff to sell some goods but this was revoked by him 7 days later which reached to the plaintiff on October 20th. On October 11th, the plaintiff accepted the offer by telegram and also confirmed the acceptance on 15th October by mail. The court held that the revocation of the offer would not be effective until October 20th when it was arrived to the plaintiff. But at that time, the offer was accepted. So, John, in order to terminate the effect of the offer he made to Monica so as to form the contract with Mark, can communicate his intention to revoke his offer to Monica. However, a condition is likely to arise when his revocation arrive late than Monica’s acceptance of his offer. Under this condition, John will have to abide by the contract with Monica and the memorandum between Mark would be unenforceable as this is the condition.
If during the memorandum John did not add the statement that he shall consult legal advice and Monica insisted that the antique belong to her, then the answer would be quite different. In the first question, John did well by avoiding legal disputes as to the formation of sales contract by stating that he shall check whether he had any obligation to Monica. However, if this condition was not included in the memorandum which actually working as a contract, John would be under the risk of breaching a contract with Monica. The offer to sell was made on June 1 to Monica and John promised to give time for Monica to think about the terms, which meant that the offer shall be binding and effective until the offer was accepted or that John revoked the offer before acceptance.  As long as John did not revoke his offer, Monica shall remain in the position to accept the offer as she liked. This means that a contract would be formed between John and Monica any time. However, by accepting the terms concerning sale of his antique without the condition to give consideration to Monica, John actually were bounded by the contractual obligation to sell the antique to Mark. Consequently, two biding contracts would be formed that Monica also insisted on the ownership of the goods.
Under this condition, John would be left with no choice but to breach either the contract with Mark or with Monica as there was only one piece of antique that can be sold. Breach of a contract means that one party intentionally refuses to undertake the contractually obligations.  Remedies for breach include specific performance, monetary compensation, injunction, recession or restitution. Specific performance is an order which requires a party to perform their obligations under a contract. In Ryan v Tontine Westminster Chambers Assoc  1 Ch 116, the court considered in a lease contract which the landlord agreed to appoint a resident porter to perform certain service as agreed in the lease. The landlord appointed a resident porter and the tenancies brought a suit which required the landlord to appoint a full time porter. The court after consideration, rendered the decision that no specific performance to appoint a full time porter would be granted.  In Stansbie v Troman (1948), a painter left the house which he was hired to decorat unlocked and the house later was burgled by thieves. The court held that the defendant was liable for the losses as this was foreseeable consequences for the failure to unlock the door.
In this case, there would be only one subject matter of the sales contract. And this means that John can only perform on of the contracts. As to the other contract which could not be fulfilled, the other party could not bring the request of specific performance. Only monetary compensation could be granted. Injured party is entitled to get a sum which can restore the position they had as if the contract was breached.  Injured party is entitled to a sum which will, as far as money can do, put them in a position they would have occupied if not for the breach.
In conclusion, if John added the condition to the memorandum with Mark that he shall wait to consult with legal advice as to the obligation with Monica, the latter contract would only take effect when Monica clearly reject the offer or when John revoke his offer to Monica. And there would be no contractual disputes. However, in the second question, John did not add such a condition. When the first offer to Monica remain legal and effective and he just entered the contract with another offeree, John ran the risk that both contracts were legal and he had to choose one to breach. And there would be consequences to breach a contract.