Selling old sails
Use ebay. I have sold many sails on ebay over the years, and have always received a very good price. I describe the sails honestly and take a lot of photos, and have at times got prices so high I feel like apologizing to the buyer! I sold a C27 sail I was literally going to throw away for a couple hundred dollars. I sold my 20+ year old main and jib two years ago for more than half the cost of my new main and jib. You won't get anywhere near the money from a used or consignment outfit that you will get by selling it on ebay.