When I've done fair business with someone and they accuse me of doing them wrong and in doing so, I owe them money. It's ridiculous. If it's buying a part, look at it the best you can, inspect it, take measurements if necessary, and look for part numbers... test it out if you're allowed. If it's a car, do a compression test, leak down test, drive it for 30+ miles, let it idle for 20+ minutes, check the oil, coolant, exhaust smell/color, the condition of the spark plugs, etc.
If you don't take full precautions and the seller didn't lie, don't be upset when something breaks or goes wrong... no matter the cost of the repair. When buying a used car, ask if there's any warranty because usually there isn't; meaning, you sign the title, it's your problem.