There are a few examples of seller protection amongst the various policies, such as protection from an item not received claim when you have tracking showing delivery, but even when clearly set out by ebay a seller sometimes has to fight to get ebay to provide the protection. 

There are other more woolly 'protections' such as for feedback, where it seems to be a total lottery what gets removed and what stays.  In your case, the buyer mentioning Ebay's involvement used to mean guaranteed removal, that no longer seems to apply.

Many of Ebay's actions/inactions seem totally unfair to a seller, and they often  are, but ebay does not promise to be fair.  The money back guarantee, for example, is very heavily biased towards the buyer.  There is absolutely no point trying to argue against an item not as described claim.  You should simply issue a return label.  

There are some things a seller can do to protect themselves, for example if a buyer sends back a different item under the money back guarantee,  but sellers may only find these out after coming to the community boards.

Essentially, sellers have to accept they risk losing the item and the money when they sell on ebay.