I used a broker I knew and trusted to help us both buy and sell. The double transaction may not be relevant to your situation, but he offered to cut his commission on the sale
since he was helping with both sides. At the end of the day, he worked much harder for less money
to sell my old boat than to help me buy the new one.
The broker's access to prior sales information, his willingness to screen
boats and his relationship with the other brokers all made the process much easier than doing it alone. The pricing and time on the market of comparable boats was very helpful in developing a negotiating strategy. Since the commission is split, there is no additional cost for the buyer. I expect that, in the rare situation where multiple buyers are trying to buy a boat, the seller's broker will try to sell to a direct buyer to preserve the full commission. Frankly, in this market, that's not going to be a common problem. YMMV, but I would certainly use the same broker if I was in the market again.