If you are dealing with a broker, he or she will have an offer form that you need to complete and sign, and accompany with a 10% check. If it is the standard YBAA form, you can walk away for just about any reason - within reason
However, I read in your previous post that there is already an offer on this boat. If there is already an offer, why are YOU looking at it? Yes, I know that the offer may be rejected. If so, then there is no offer on the boat. DON'T let the broker use you for leverage.
Ask the broker if the other offer is still being considered. If it is, tell the broker that you need to look at other boats. The broker should only tell you about this boat if, and only if, the other offer is rejected. If ANYONE has the right of first refusal, it puts both buyers at a disadvantage. I would only enter into negotiations if I were the only one making an offer on a boat.
I had an experience with another member of sailnet where the other member was selling a gorgeous boat, that I would have loved to purchase. However, he told me that another person had made an offer, and he granted that person the right of first refusal. I never made any offer for that boat, and have no regrets. I have nothing but respect for the other buyer, and the seller of that boat. I can only hope that they
respect my decision not to initiate a bidding war.