Over a lot of years of sailing,
I think I've dealt with 3 of the 4 sail brokers listed below -- and was happy with the sails I bought from them. In buying a used sail, it is best if you deal with a nearby broker and can inspect the sail first. If you cannot, be sure that you can return it for a full refund if, for any reason, you are unhappy with it or it doesn't fit right. Begin the process by being absolutely sure of the sail measurements that will fit your boat. Do not use stock sail plans or measurements that were used on another boat. It only takes a few minutes to measure your boat for a sail so don't shortchange yourself. Finally, understand the meaning of sail condition used by the broker. I believe, for example, that Bacons (which is near me and I've bought a number of sails from them for other boats)tends to underrate their sails. That is, a sail they rate as "Good" would be one I might rate as "Excellent."