We say it here all the time; caveat emptor.
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yeah, as you know, even people that are honest don't always know everything there is about their car if they bought it used . . they might have bought a rust bucket that the bondo hasn';t fallen out of yet so they dont know its full of bondo.
some helpful things to do are:
have the seller take a video of the engine idling.
video of the tail pipes as he revs it up.
video of him flooring it from a stop.
look for smoke from the exhaust and listen for odd noises.
take several GOOD photos of the bottom inside edge of the doors.
lower inside corner of the doors.
rear door jamb in the body.
outside of lower trink extension.
rear frame rail mount as shown earlier.
top of shock towers . . the lite blue f/b had a big patch on top.
floor pan.
photo down the side . . you can see by reflections if its straight or wavy this way.
miscellaneous photos.
if just those few underside photos are good, there is little need to be concerned about rust anywhere but few cars pass all those tests these days.
if the buyer says he's too busy or doesn't have a camera, then i am too busy to talk to them anymore.