Demand for used surfskis and outrigger canoes can vary greatly depending on your location. If there is high demand then it’s a seller’s market and used boats typically seem more expensive than they are in other places where there seems to be a greater supply or low demand. Some of the resale websites listed below may not get a new listing for a long time. Notice also that many of these sites will allow you to post what you are looking for, not just something for sale. It is a really good idea to talk to local boaters who can keep an eye out for a used boat for you. They may also know how to get the word out to other local paddlers via local club email and paddler blogs. Talking to local dealers and brand reps can help too (if they are willing), sometimes they hear about boats that are only sold via word of mouth - which will typically be sold before you otherwise ever hear about them unless folks in the know are watching for you.
For facebook sales groups - you can try methods like doing searches for “surfski” within facebook groups to narrow down what you are looking through, but be aware that some ads for surfski sales just mention the boat brand and model and do not even mention “surfski”.
From The Adventures of Rocky and Bullwinkle and Friends:
Rocky: We did - the front page in the want ads, see? (reading newspaper) “…Moose will sell Ruby Yacht of Omar Khayyam to any interested party.”
Bullwinkle: I said I wanted to SAIL the Ruby Yacht AT an interest-ING party!
Rocky: Well, that's the press for you.