What kind of documentation for used engine would satisfy you?

Show a video of it running. Have all the documentation you can. DO NOT drain the oil and remove the Filter (ask the buyer what they need in regards to that.) Show the Gauges in the video and a walk around of the exhaust.

And lastly, leak it down and show each cylinder. Anything else otherwise (for me) is considered a boat anchor. A Leakdown will show the condition of the motor short of a mechanical issue which will show up in the oil.

That may be more than you want to put into effort wise but you asked and since I got popped on a motor 2 years ago (my stupidity for trusting someone) that's what I would require of someone.

JW