I have been selling on ebay since late 1997. I sell collectibles, some vintage and have had some problems with bidders telling me the item never arrived or arrived damaged and being a 'Newbie" I promptly refunded when they sent the item back.
Until this one time, I received an item I knew wasn't the piece I sent to the bidder. It was a ceramic piece that I had taken special care to clean. The bidder sent back one that was broken, dirty and smelled of smoke (smoke free home here). What to do??? I let it go, but I resolved never to let it happen again. I revised my shipping and return policies to add that all my items are invisibly marked and why I was doing it. Then I searched and searched for a marker for that purpose. FOUND ONE on ebay!!!!!!!
The marker is used for detecting phony money but works on ceramics and paper. You use a blacklight to see the mark which is invisible to the naked eye. Do a search in ebay under "security marker." There are a few listed but the one I use has a yellow pen and a small silver light that looks like a very large bullet. Great tool.
I haven't had a problem since I've begun using this. I did have someone recently email me that an item was not as represented and she wanted her money back. What she described to me as the damage, I knew she was trying to switch items (I take very detailed pictures) , so I told her she could send it back, but before she did that to check my retun policy on my ABOUT ME page. Next thing I know she emailed me and told me never mind, she decided to keep it for the price she paid.

I blocked her from bidding again in my auctions.
As far as people telling me the item never arrived, I send everything Delivery Confirmation. I can tell them the day it arrived and the time of the day it arrived. That 45¢- 55¢ has saved me a lot of aggravation. I build DC into the shipping/handling etc., that and insurance are required on my auctions and are not negotiable. Stated right on my auction page.
Every seller sets their own terms and policies. You have to figure out what works for you and then tweek it until you are happy with it.
Hope this helps to aleviate some problems.
