Know I'm going to get some different opinions here, but thats ok.....here's mine. I would avoid almost any used airplane engine unless you have some pretty good knowledge of its history....and I don't mean the log book. First thing you have to ask yourself is why is this engine available for sale. Why would anyone remove a perfectly good engine from an airplane ? It is possible that someone wanted a new engine and pulled the old one. Still doesn't necessarily tell you anything, because he may have knowledge of something that the log books don't tell. If he has plenty of bucks, and cost isn't a factor to him, then hey he just wants a new motor because he wants a new motor. But 99% of motors don't fit that scenario. So why is this used engine out of an airplane and up for sale? Most likely the airplane it was in was either destroyed in an accident or allowed to sit and deteriorate till it was better to part it out than to fix it. If it came from an airplane involved in an accident, there is a good chance there was a prop strike. So ask to see the propellor that was on it. Buying a used engine is a crapshoot at best unless you know specific info on the motor. A cheap price is usually an indication that you need to look deeper. Not having the ability to properly inspect an engine means you need to pay someone knowledgeable to check it out.......not cheap and not always possible. Now if you are going to pony up and buy a new or "factory" rebuilt engine, thats different. You will have a reliable engine from a known reputable source and most likely no problems. Know this, whatever aero engine you might buy, it will not be required to have the same annual inspections that would be required on a certified airplane. The thing is, if you expect to have the same level of reliability that these engines provide in certified aircraft, then you need to perform those inspections even though they aren't legally required. ($$$$) All that being said, many people buy used engines and get good serviceable engines. Many other people buy used aero engines and have terrible and expensive results. So if you decide to venture into the used aero engine world, have plenty of money in reserve and take a good AP along with you.