Why not write up a legal contract covering yourself and the seller?
or
Would a plane ticket cost so much to fly out for the day or weekend to make it a deal breaker? Buy it in person, take it with you to the closest freight terminal and ship it.
or
Ask the seller for references...
or
Ask the seller to take it to a local machine shop to him of your chosing for inspection prior to purchase?
or
Buy an engine locally that you can see.