I assume the buyer has opened a case and claimed that the item is not as described.
You can contact Ebay Customer Services but they will say accept the return and issue a label to get the item back.
When you get the item back, if the claim is, in your view, false, you can refuse a refund. Ebay will probably refund the buyer if they ask Ebay to step in but you then have the right to appeal. If you appeal, you can show new evidence to Ebay. It is all heavily biased in favour of the buyer.
Really, the best thing to do is get the item back, refund the buyer and sell the item again.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)