I bought a 94 Mustang 5.0 HO with the computer, harness, and all of the parts to make it run for $1500 from Denver Ford Parts a few years ago. It was and still is gem. I got to hear the car run and verify the milage of 64K before they pulled the engine. I got to see the damage and why they scrapped the car. I've driven it in my truck for 3 years now and its the best starting, most reliable, and trouble free vehicle that I've driven.
I told Larry to find me another one just like it for my 70 EB project. He found one in a wrecked 94 Mustang GT. I picked it up and brought it home and set it up in my test stand. Took a while to get it fired up and started trouble shooting. It turned out to be a junker. A $1500 boat anchor! ODB self test showed 2 weak cylinders and a bad ERG valve. Compression ran from 85 to 120 lbs. The rubber on the harmonic balancer was poocked out and wabbling. The rear seal leaks and pan gasket is pooched out on one side. They had cleaned it up pretty well and I didn't notice the problems until I got it home. Heart surgery prevented me form dealing with it for about 6 months. To be fair, Larry would probably have swapped it out with another if I could have returned it. They did tell me that they had run the engine, it sounded good, had good compression, and was not throughing any codes. The old guy that pulled it even told me that he had put it on the exhaust analyzer it was running clean. This was impossible given some things that were inoperative on the engine. The plugs were frozen and hard to remove. Hard to do a compression test without pulling the plugs! Two plug wires were reversed and looked like they had not been moved based on the way the ends stuck in place. The throttle body had been removed and they had to come up with another one that was obviously not run on that engine based on the dust and dirt inside of it. I assume it had some aftermarket one on it that was pulled before it was junked. The bolts didn't even match.
With all that said, I'll never buy another used engine that I can not observe run.