someone is selling a used engine and they say it "has strong compression and still has a tight pinch" and the picture shows the piston sitting at TDC?

Do you mean its beyond tdc?

How can an engine go beyond TDC??

AAC (aluminum aluminum chrome) engine pinch so tight when over heated the stretch the piston. The when they cool they're about 0.005 thinner and slightly above TDC.

personal I wish some engine maker would make AAC, but c for ceramic there so smooth.
(Norvel .25 15,000 rpm on the ground with 9 x4 prop AAC ceramic )

my point is anyone who knows how to maintain there engine always keeps the piston at the very bottom when cooling off or when not in use.
i will not not buy a used engine when it has been sitting aroung with the piston all the way up at top unless i plan on having it pinched.

Nice info but far from real. It is all depending on the used materials of the piston and sleeve. The piston has a high level of silicon (up to 30%) for more strength and less expansion. The sleeve has a larger expansion rate so in cold conditins you do have some pinch which also helps a cold start. Because the sleeve gets more cooling than the piston there will be a ideal fit on operating temperature.
If you choose a piston with a higher expansion rate then in cold conditions the piston will not build up compression but on temperature it will have the ideal fit, this is not what you want.

The ideal situation is to use materials with no expansion, after running in it will have the perfect fit on every temperature and no heating up or perfect operating temperature is needed.
But that is not possible.... although sometimes you get a perfect engine with no cold pinch which is easy to start and can be brought to operating temperature in a faster time.

I had one of those. The very first AAC Sirio .12. That thing when cold could be spun over like a pan car axle with no motor. I swear it had no compression at all. Despite this, the damn thing would still start. It would run kind of rich for a few laps but once it warmed up (never more than just a few laps) it would outright scream. Won a few nice races with that motor. I then sold it to someone who sold it to someone, etc. etc. Now a buddy of mine has it and still runs it in the occasional dirt oval race.