BEFORE you accept the return, ring eBay and explain that the buyer made a mistake assuming it had a specific function etc, and that the return request is a Change of Mind. Ask eBay to look at the messages from your buyer as they can read all eBay messages. They will be able to change the return to Change of Mind instead of SNAD. You won't have to refund any postage this way.

I HAD. Return request for SNAD but turns out the buyer made a mistake, sent back a different item and got a full refund of $105 inc postage and kept the original item. When I rang eBay to complain they advised that if I had've called prior to accepting the return they would've been able to change the dispute to change of mind, which would have meant no refund on postage.