I've bought 3 private party and 2 brokered boats.
There may be more wiggle room on price if a private party doesn't have to pay the 7-10% brokerage fees.
Hopefully the broker is also a dealer that can scan and verify hours, facilitate a sea trial, help with some paperwork, etc. Maybe they serviced the boat in the past? Sold it new? At the same time they could know absolutley nothing about the boat. Broker may have to collect sales tax.
One of the brokered boat I bought, the dealer was helpful in tracking down a trailer title. Boat was in a title state originally, being sold in a non-title state. I flipped it and was going to a title state....it was a mess.
In the end it's up to you, the buyer, to cover your bases. Get a survey, arrange financing, a test ride and cut your best deal.