I don't think I'd buy a used engine without dynoing it. If it performs as the seller claims I pay for the dyno. If it has any issues or doesn't make the claimed power(within reason) then the seller pays for the session. There are too many people in this sport/ industry that are incompetent/ liars/ scammers etc, to risk that kind of money on used engines.

At a minimum it should be run on the floor for you!