i've bought and sold a lot of car audio over the years. most of my systems were paid for from the proceeds of buying cheap, testing/fixing, then selling. i sell by taking such detailed pictures - and pictures of measurements, that the buyers have a high confidence. i'm also honest and don't receive complaints. everything is as advertised. if i'm selling an amp and you want proof it works - i'll scope it with a dummy load.
what it boils down to is that there are scammers everywhere -
ebay, CL, forums, etc. you can weed them out by being careful in your purchases. CA.com has rules for classifieds, but the rules don't state pics of proof of function. i.e. use a DMM to measure the coils or output of the amp while powered and fed a signal. if don't own a DMM i won't buy audio from you anyway. and if you can't take those pics then you can only ask for 1/2 the value of the product.
i have a test bench setup, i can fully test amplifiers/speakers/headunits/eq's/etc. but it's time consuming and you always take a risk.
over the years, i've had a few DOA's. sometimes you work it out with the seller, sometimes you are screwed. it's just the risk we run when buying used from strangers online. usually, forums are a bit safer when buying from regulars and the feedback systems can help raise flags.
lastly, call the person. talk to them. if you're gonna buy something, have a conversation with them about it. you can usually figure out if they are sincere pretty quickly.