In Article 4 of Part II: Relations with Clients, in the Certified Professional Yacht Broker (cpyb.net) Code of Ethics, we find this answer.
In the event that more than one offer on a specific vessel is made before the Seller has accepted an offer, all additional offers presented to the Broker, whether by a prospective purchaser or another Broker, should be transmitted to the Seller for his consideration. In the event that a Broker, Brokerage house or central agent has received more than one offer prior to acceptance of any offer, all offers should be presented to the Seller. The Broker shall act on the instructions of the Seller as to which offer shall be accepted and/or negotiated. If an offer is made after owner has previously accepted an offer, the owner should be made aware of its existence.
This outline is a recommendation for a fair procedure for Brokers to follow when several offers are presented at approximately the same time. The procedure for handling any multiple-offer situation should be discussed with the boat’s owner. Ultimately, it is a Broker’s obligation to act as the owner desires and by whatever guidelines he decides. Whatever solution is decided by the owner, all those making offers should be apprised of it.