hey John, Well, I get used engines gave to me mostly and the engines arent generaly hurt in any way just whatever they were in, trimmers blowers, chain saws and the like have been broken in some way but the motor is fine. When one converts these motors the first thing to go is the carb and sometimes the muffler too. Some people are buying new homelite and ryobi trimmers for $70 or more from wal-mart and for as little as $50 from harbor freight. For me, since I have to put a larger carb on them anyway why buy it new if I can get good used ones gave to me and spend the money on a larger carb? This becomes the problem you may run into trying to sell your motors and get at least $50 out of them. Many people that buy the new trimmer are removing the motor and selling the leftovers on ebay for 30-40. There earning most of there money back this wayand ending up with that almost free motor thats new.Look around a bit and youll find them doing this on ebay. If you fully convert the engines you have for aircraft use and have them really ready to fly I would think you might get up to $100 for them a peice but it depends on who happends to see them on ebay. I would try to sell them as is first. Heck John, the reason people convert these engines anyway is because we dont want to spend any money and really like tinkering on this stuff. You can bet if I was making all kinds of money I would buy high end gas motor made for the hobby and just bolt them on and enjoy the no hassel power right out'a the box.