I am a fan of working through a broker, most sales end up involving a broker, so you end up paying one whether you use his/her services yourself or not. A good broker can save you a lot of walking around time, looking at trashy boats or boats that don't meet your needs or expectations. See if you can get broker recommendations from recent boat buyers, walk the docks at a marina and ask owners if you don't know any. Meet with several of the recommended brokers, talk about what you are interested in, see how interested they are in helping you, then ask one to look for your boat.
A good broker should be able to identify those brands/models that will work for you...If he/she doesn't seem to attentive, or on the mark, move onto to the next broker, eventually you should find one you enjoy working with and trust. Don't trust too fully, remember the broker is the sellers agent, their loyalty is divided at least. The one you work with does have a vested interest in treating you well, or you will just move on...just remember how they get paid.
You never need to pay a broker directly, for anything, they will get a fee from the sale proceeds, if and when there is a sale..
PS - as to establishing value, there arer some guides like
Used Boat Prices – NADA Guides Boat values, Marine prices and Outboard Motor prices and brokers can review completed sales off
New and Used Yachts for Sale - YachtWorld.com, the problem is there is a wide variance in boat values due to condition, upgrades, and equipment that the guides only sertve to get you into the ballpark of price. Each boat needs its own inspection to figure where it might fit on the wide value scale. My personal opinion: boats with comparably low asking prices for that model are more likely to be worth even less.