I sell in several online venues and have a network of friends who also sell online.  There appears to be a popular scam with some dishonest buyers. 

The buyer will buy your mint or new condition widget and then claim that you sent a damaged or broken widget.  They will want to return the widget at your cost and receive a full refund.   The widget they want to return is an identical widget but in damaged or broken condition.  In this way, they can replace their widget for your mint or new widget for free. 

These scammers know that filing an insurance claim will not work because the carriers will not approve a claim for a widget that was packed carefully and looks to have old damage. 

How can you protect yourself from these dishonest buyers?  Very easily!  Buy an invisible pen and UV/blacklight.  You can find many for sale on ebay, online, and at super discount stores.  The price is very reasonable. 

Choose a mark that won't be easy to replicate. Your signature is a good one.  You can even go as far as to have non-relative witnesses to your mark and have a statement notarized, but this rarely necessary.  Take the time to mark all of your widgets before shipping.

When you encounter one of these dishonest buyers who claim your widget was damaged before shipping, politely reply that you disagree because all of your widgets are carefully inspected several times before listing and shipping (ensure that you do this).  But because you want to give good customer service, you are willing to accept your original widget returned in the same manner it was shipped.  Once you are able to verify your mark, you will be able to refund.

Collectibles, glass, and electronics are extemely vulnerable to this type of scam. I have not had very many buyers with this scam and neither have my friends, but between us all, the attempts are appalling.  Yet once the email goes out concerning the invisible mark, not one of us have ever heard from these buyers again. 

This is a very inexpensive and effective way to protect your investments.