I am suspect of anyone who does a motor replacement/upgrade and then turns around ands sells it. There are alot of them.....I would suspect engine/trans cooling issues on these.
While it does pay to be cautious, there are many other reasons to sell a car with a fresh engine rebuild.
One reason was I was simply bored with it & needed the $$$ & garage space for the next project. A # matching '68 4spd SS396 chevelle (should've kept that one
Another was a nice street sleeper '82 cutlass supreme that did the 1/8 mile @ 100+ mph in 7.20sec. I would have hurt myself if I'd kept that one much longer, fun while I had it tho . . . .
Then there was the '69 SS396 chevelle that I sold due to job instability issues. That's the one I regret selling the most, it will cost me 10x what I sold it for to get another.
So, not every seller is a con artist looking to scam everyone, some of us just don't think that way.