Papso, Thanks for taking the time for your lenghty reply, with helpful information. "Essentially, sellers have to accept they risk losing the item and the money when they sell on ebay." Yes, a grim prospect indeed.

To give a bit more context I will quote from the help pages-

"If you don't respond to the return request, we may refund the buyer and seek reimbursement from you, without requiring the buyer to send the item back." So the buyer gets to keep my brand new £600 lens which he claimed had dust specks in it, and I am left with nothing. (So I had to accept the return, but On return of the lens I cannot see dust in it) This is ike me going to a camera shop, buying a new lens, claiming it has dust in, which the staff in the shop are unable to see, and demanding a full refund and to keep the new lens.

"If we're asked to step in at any stage, you'll no longer be able to deduct an amount from the buyer's refund, even if you qualify under our seller protections."  So my fully workng £190 camera lens is returned to me broken because key spare parts have been harvested from it and I am left with a junk lens. This is like me asking a policeman for help because I have been robbed and me being punished for daring to ask him for help. I have asked Ebay agents to talk me through how this works in any generally accepted framework of what constitutes right and wrong, fairness, decency and honesty in this world, without receiving answers that give me any reassurance.
(I had to accept and am waiting on the return of this. It is clear from the buyer's own listings that he modifies camera lenses for video use. This is hypothetical at the moment)

"Here are some guidelines to help you determine how much to deduct from the buyer's refund.
Missing parts - 15%–30% deduction
Missing essential parts,Item is damaged, scratched, defective, or requires service or repair 35%–50% deduction"
So once again I am out of pocket.