Built motor is hard to inspect. If race spec, you'll hear some rattle from the pistons.
Owner should know who did the work. Have the shop confirm the buildsheet.
You save a lot by buying the car built as you only get a fraction of the build cost back when you sell. That's why many cars gets returned to stock prior to sale.
He told me who the shop built it was, but I was wary of having the shop being biased with the seller if there were any hidden problems.