Buying used parts

Bought and sold lots of parts in past...on convertors only if I know the person and the car it came out of,cams,is why I carry a digital micrometer so I can mic them and chk them out against their specs plus a visual on everything in general,fuel pumps if they are clean ,look good and work[electric ones] yes even at a swap meet I will ask to hook them up to chk the input/output pressure...intakes just a visual..carbs if they seems to work[clean is a factor] and all parts are there and can be had for a decent price[usually figure it needs a kit as a rule of thumb] or I know the person...electronics only if I know the person or is new in box,rocker arms usually just a visual inspection along with the same for pushrods...pistons, rods, same visual and the person selling them unless new in the box..gears,3rd members depends on a good inspection and the person you are dealing with,same with blks and heads...alot depends on 1st..visual, 2nd. the known dealings or trustworthyness of person that is selling 3rd you must have a general decent knowledge and experience of said items you looking at to understand what is good useable or worn out broken pos..and price and if there is some kind of guarantee or refund/replacement[known people or dealings on the net] Yes there is always some risks and yes I have been burned a few times but overall I've had pretty good success,just have to be careful...i have only gotten one bad convertor and was from a friend and a known car that I pulled the part out of that was working just fine[neil chance 4500] but when we got to chkg it found out that had a hairline crack where the snout goes into front pump almost didnt see it,was clean fluid,looked good known to work good but I couldnt sell it as a good one knowing about the crack so it still sits on the shelf in my garage and my friend just gave it to me no charge..was just out my labor,it can be fixed just have never got around to it, got a dynamic 9.5 that I wouldnt trade for any of the others...but most time in a perfect world where money is no prob buy new..lol..but new doesnt always come trouble free all the time either..you live and you learn...