Over the years,
I have bought a number of used sails for some of the boats I've owned. I don't really think there is a "best" place. I've purchased sails from eBay, on this site (Gear for Sale), and from Bacons in Annapolis. To me the key is knowing what you want and what will fit. Getting the right fit for a mainsail is difficult and unless you can find one specifically made for your boat, buying a used one could be problematical unless you can return it. Headsails, on the other hand, really only have one critical dimension, the luff. I've found that asking for pictures from private sellers is useful. Getting a full and complete description of the sail and its condition is critically important. I continue to be astounded at the number of people selling sails who have no idea just what a sail is. I've had good luck using a sail broker like Bacons because it is only an hour away and I can inspect the sail there before buying, and can return it for any reason within two weeks of purchase if it's in the same condition that I bought it. No matter where you buy a used a sail, I would always ask for the right to return it, usually at my expense, if the sail doesn't fit or is not as described. Caveat emptor....