I would take it the have opened a dispute vide the new eBay Money Back Guarantee.
The process requires that the buyer first attempts to resolve the issue with you (the seller) and they have three days in which to do so. Now as I understand it, you say the description was correct, and that the real reason for the return is that the they โ the buyer - simply bought the wrong part.
So, in the initial stages I would recommend you attempt to resolve the dispute by offering to accept the return as long as they - the buyer - pay the return postage and on receipt youโll refund them the original sale price only (not the original postage) plus a further 10% reduction to cover eBay and PayPal fees incurred.
Now if the buyer refuses the offer and escalates to a claim, then keep the response factual. That is, that a refund under the guarantee is limited to circumstances where the item WAS NOT AS DISCRIBED in the listing, and a mistake by the buyer in choosing the wrong part doesnโt fall with the description of NOT AS DISCRIBED.
Finally, make sure everything is done I writing, and if eBay decide for the buyer, simply lodge a complaint with the ACCC on the basis that, either there is a systemic problem with Guarantee rendering it invalid, or that they, eBay, are refusing to abide by the limitations imposed by their own agreement by extending the ability of a buyer to return good under Guarantee to include returns that do not fall within the definition NOT AS DESCIBED
