Document the crap out of it.. I sold my LG Voyager on ebay. Some guy tried to rip me off saying it wasnt working. (phone had 32 minutes on it.. brand new)
I had taken videos of it powering on, touchscreen working, dialing and talking.
basically everything a person would do on a normal day..
He filed a complaint with ebay saying it didnt work at all, I presented my proof in the form of youtube videos. Ebay's safeharbor ruled in my favor.
Yeah, what he said, but on top of that there are lots of tools that ebay offers that can help you assess a buyer or seller. I've never sold anything on ebay, but I've bought a lot.
From a buyer's perspective, I only deal with sellers that have a 99+% user rating. I only use Paypal and I generally don't like to deal with a seller that only has less than 10 user ratings. I also prefer a shipping method that can be tracked, unless the item is under $10. I figure I can lose $10 without being really upset. Priority mail is good for shipping and I've seen tons of sellers now using that because the cost is dependent only on which box you can fit your object into...therefore, it's a fixed price not one based on weight and destination.
From a seller's perspective, like the dude above says, document your object. Also, I've seen sellers who won't deal with any buyers who don't have more than a certain number of purchases under their belt. I don't know if there is a specific tool for this when you list your item, but I'd assume so. Also many of them will only use Paypal, as well. And once again, use a shipping method that will track the item so nobody can claim they did not receive it. And, lastly, you can stipulate no refunds whatsoever or whatever you deem reasonable, and also offer them an additional fee to insure the package in case of damage. This puts the onus on them if something happens to the item during shipment and they didn't opt for the insurance.
As long as you state all your terms in your listing, no matter how tight they might be, you've got a leg to stand on should you get a dodgy buyer.