eBay gave you incorrect advice as private sellers must refund the whole payment. Only registered business sellers can make reductions for damage allegedly caused by the buyer.
I'm afraid your situation is what happens all the time. Sellers often complain that buyers have damaged their items, or even returned a different one than the item sold. eBay guarantees the buyer a refund if an item is not as described, but as they never even see it they often just take the buyer's word.
Sellers can appeal eBay's decision, and report the buyer for abusing the guarantee, but the case is still liable to be found in the buyer's favour.
This is one of the reasons why eBay is a risky place to sell things.