What kind of documentation for used engine would satisfy you?

Just another thought, you could build a cheap run stand, and mount it up, store the engine on it instead of a pallet and wire up an ignition and a battery. Then when a buyer comes along you could fire it up for em. I built my run stand for like $30 bucks worth of 1-1/2” square tube and some biscuit mounts.

Not a bad idea. I may have to invest in a cheap exhaust system with mufflers so my stuck up neighbors don't call the police on me though! :D