sceanario: sell a piece of good gear on ebay. buyer gets it, swaps it with a broken unit (or breaks it himself) and returns it. even worse, he could return a box of rocks and as long as it has the right weight and was mailed to the 'right' address, paypal will not side with you.
just too much risk. if there was a trustable escrow, that would be great, but I have not heard of that for years. I did use an escrow once when I was buying a $1k used camera body, but that was nearly 10 yrs ago.
buyers have more rights than sellers and you have to hope you don't run into a bad buyer. most buyers are probably fine and most sellers are, too; but the risk is just too high for several hundred dollars of gear, each.
also, paypal can freeze your account and hold your money for as long as they want (in the US). they are NOT a bank and can get away with murder. if I'm buying, I don't have funds in my paypal account, but if I'm selling, I would; and I simply do NOT trust paypal to 'watch over' my funds.
when ebay was a garage sale style site, that was great. now its mostly about power sellers and cheap trinkets. the nice multi hundred dollar test gear is not what ebay is about these days; they are about selling cheap junk from china and the small time seller gets no respect from paypal or ebay.
I can't average out my losses and so if even one of my meters or supplies gets stolen by a bad buyer, it would crush me. I'd rather keep the gear and not make money from it than risk being an ebay seller, to be honest.