In my mind, unless you own a boat which is a pretty strict one design, or you do not care how your sails fit, then its makes next to no sense to buy a used sail no matter how cheap it is. A well made sail is more than a simple white triangle. It is cut to the expected headstay sag, mast bend, hardware geometry, the characteristics of the boat such as its need for more drive or pointing ability. So even if you succeed in finding a sail with roughly the right luff, leech and foot length, its not so easy to buy a sail with the right geometry to work with your boat.
If your main objective is to sail around bouy's faster than the other guy, then used sails won't do, this is true. If sailing is a fine tuned "sport" for you, and every nuance of sail shape is what drives you to sail, then no, used sails are not the way to go. You'll also want a hull that has put speed above saftey, something light weight in construction and you will want a light anchor so as not to slow you down. However, if sailing is a means of exploration and you want to safely get across the ocean to see whats on the other side, and your the kind of sailor who will purposely dump wind to slow down in order make the harbour entrance after sunrise in stead of 3 hours before, when the racer dude is raising his aysym. And If you are the kind of sailor who is reefing to lessen the strain on the rig and hull, when racer dude is heeling and yelling "yahoo, we are sailing now!", than used sails are the way to go. like I said, I bought a like new, double reefing main, a storm jib, a working jib that fits like a glove, a 135% genny that sits perfectly and a black yellow and red asymetrical in excellent conditon for 1400 bucks total! And I am sailing to Honduras with an affordable, good looking ,new to me, set of sails in less than a month. This left me money to buy a fancy new wind vane, a bigger better life raft, a g-pirb, a sat phone, the bigger solar pannel I wanted, a spot, new fouly's, a back up auto helm, all new blocks, main sheet, halyards, a vhf with ais, and a new gps. All which I would have done with out if I had gone for all new custom sails. Which I do want. I have sailed aprox. 15,000 miles of open water on just my little sloop alone, to as many as 10 different countries on good used sails, with no engine. Next I buy the new mast, then the engine, then I get the local sail loft to start sewing.