A used solar panel would be about as hard to dispose of as a used 100 dollar bill. They don't get to see the light of day in a public forum unless a project renewal takes place and a dealer snaps them up in a lot sale.
"Damn, tighten it down wrong just a little and it don't do squat. Better sell this turkey. Standing up or laying flat, loose, it tests fine. Someone will buy it"
You have no guarantee you won't run into the same issue as one of my customers with his used panel. Of course the "sale" was made in a Denny's parking lot "for convenience". Then again you may end up with a good panel with just the slightest of ricochet scratches across it, and pigtail wires ready to solder to.