But do we buy a used car and disassemble the engine before we drive the car? No....
I'm not saying buy it sight unseen but a good inspection without disassmbling the engine is possible.
I bought my race car motor blown up... pulled one valve cover and the pan to inspect the carnage. While there was some unseen damage I bought the complete engine carb to pan for about what the crankshaft would have cost new from Mopar. Perfect heads. Block had two minor nicks at the bottom of the cylinders. Crank was easily repaired.
In the racing world engines are sold everyday racer to racer without a teardown. The sellers word often is good enough. Do some guys sell junk? Sure. Last summer I was looking for a SBC for an older dirt late model I have and want to sell. Found a long block for $800.00 from a local racer that had name brand parts and low laps. One call to another driver (his brother) made me back off on the purchase. The engine had good parts yes... but it was a mix and match conglomeration of parts from a few blown up/worn out motors.
A good DLM motor can be $30,000 used!
Do your homework but pulling a valve cover and an oil pan may be enough to tell you what you need to know.