I have attempted to sell a couple saddles on a few different websites with no luck. Although they would take a commission, I would strongly suggest looking into Pelham Saddlery or Trumbull Mountain.
Its not free but...I had a good experience with fineusedsaddles.com. She does a lovely job marketing her used saddles, and specializes in the higher-end names. I'd definitely look to her to buy my next saddle too.
This was a timely thread for me. I have a high-end British-made saddle with a little-known brand name, with very (VERY) short flaps that I need to sell. It will be a challenge, but with all the info that Patricia puts on that site, I think she can get it done. When I get the serial # and all the other info she is looking for, I will contact her too. Thanks everyone.
"You become responsible, forever, for what you have tamed." - The Little Prince