Sailingtexas
ebay
sailboatlistings
iboats.com
sailboatowners.com
and craigslist seem to be highly favored for the under $20k boats.
I've used each of them to list my boats. I've bought, and sold 3 boats in the 22-26 foot range in the last 4 years. I won't say which ones I had the most success with, but each of the listings above were VERY helpful to generate interest.
That being said, I ALSO contacted boat owners through brokerages, both using yachtworld and independent brokers some attached to Marinas.
I have NOTHING but favorable things to say about each broker I dealt with, and each knew their owners, and their boats quite well. The sell was not "hard" nor was it sugar coating. The warts and the bonuses were pointed out. The brokers were quick to follow up, and get back with questions from their owners.
If I were selling a boat under $20k, I'd attempt to list with the free/low cost sites first... if I started to become pressed to get it sold, I'd involve a broker in a heartbeat. They know how to sell, and know how to advertise your boat properly (including shotgunning all the places to list boats). They certainly earn their commission.
If I were buying a boat, I'd not hesitate to purchase through a broker. I'd actually prefer it. The free/low cost sites make this process much caveat emptor (and I assess the seller with probing questions to see if they know enough what they are doing to justify my usually long drive to go see a boat). Even with that, I've been burned and wasted a day driving. Most private sellers are quite good, and are awesome to deal with, but some are downright scum, and would show you a sinking boat and try to tell you it's recently "washed." My favorite was one that told me his boat had a road worthy trailer ( when he KNEW I had a trailer that fit his boat already that was registerred and working), and I got there and it had spent bearings and 2 flat tires (there was a plethora of other things wrong with the boat too, and he feigned not knowing, except his "uncle" who showed me the boat was a certified boat inspector). If they hadn't taken me on a nice tour of the thousand islands while I was there I'd have tried to have his license called in question.
When I listed my own boats sold on the low cost/free sites I made sure to point out the problems with my boats, as well as the good points. I wanted anyone to know exactly what they were buying. Then I went OVER the top if they worked with me to help get the boat to them. My boats were trailerable, and I offered what I could to help get the boat home (delivery or partial delivery was involved in my last 2 boats, negotiated with the cost). I also insisted on cash for purchase (this requires the buyer to make sure they do the legwork first - as you have to be serious to have cash in hand to buy). I made sure my buyers could see the titles of the boats I sold, and I did the title search for the ones I bought myself.
Hope my experience helps in some way.