So far, I've been lucky in buying used engines. I haven't been burned yet. The best advice I can give is buy local and know the previous owner. That being said, I just bought a used Saito .80 off Ebay and I'm very pleased with it. In my RC experience, I've found most modelers to be honest. Watch out for the guy who doesn't know much about RC or the equipment.

Check the engine for good compression. Shine a flashlight into the exhaust ports and check for scoring or burns. Listen for noise and feel for roughness while rotating the crankshaft. If you're considering an online purchase, look for pictures of that particular engine and not a stock picture from a catalog. Ask the seller for additional pictures. Include specific requests such as closeup of exhaust ports, etc.

Dr.1