I'm ususally a good judgement of charcter, and can tell if someone is lying or acting desperate, telling what they think I want to hear to make a sale. Paperwork, compression test, and oil pressure tell alot about a used motor. So does looking at gasket edges, colors of brass freeze plugs etc can tell if a motor is actually new, and if so, how new or old is it. I say all that, but I've always built my motors.
1 motor I did buy once for $150, the old guy kept telling me, now it ran good all the way from Ardmore, but it smoked when I started it up, now it's an old motor, but it smokes, now this motor might not run for very long, LMAO, I finally said, you don't want to sell it do you? He said, NO, I better not, I just better junk it, I'm afraid it won't run for long and I like to sleep at night. I said, I'm just buying it for a core, I intend to rebuild it, so I don't care if it runs or not, but just so long as the rods are good, no holes in the block, and a turnable crank, that's all I care about. He said, Oh, that's all good, you can have it for $100. I loaded it up, it sat in the garage untouched for about 2 months, them my bro blew his motor and needed something NOW that ran. I told him, well, this is one is supposed to run, but I bought it used as a builder. Bro, I have nothing to lose by trying it. Sure enough, it puts out a puff of smoke when he starts it every morning, add a qt of oil every month, but he's been driving that motor in his El Co for over a year now, and goes back and forth from OKC to Ft Worth at least once a month. Yep, it's been a great $100 motor.