I semi get why you say this.
But its not much different then from buying a used rc from a random guy.
However usually you can talk to the random guy, ask question about the car and its condition so yea.
a pawn shop just wants to sell. and any information they give could be false.
EDIT to...
i small trick to truly know if compression is good is to lightly grab and lift the entire car by the pull start, dont swing it around or pull hard on it.
if the car can stay air born while you hold the pull start only, it has great compression. if it starts to dip down slowly you are still okay...