boat us also has a valuation service. very informative. surf their website.
I have seen brokered boats NOT listed on yachtworld but on other websites. Surf all the for sale websites.
Some sellers sign an exclusive agreement meaning the broker gets a commision no matter. But other brokers will allow sellers to find their own buyer and avoid all commission!
Here is one site that has a lot of by-owner listings Sailboat Listings - sailboats for sale
Also surf the specific boat owner group websites. If the boat is fairly popular it will have an owners group and probably have a for sale by owner section.