Buyers have 30 days to return an item that they consider is not as described under eBay's Buyer Protection/Money Back Guarantee, and if your buyer has already opened or opens an eBay item not as described return, they'll be required to send it back to you for a full refund of their original payment, and you'll need to provide them with a pre-paid returns label (if you don't provide a label, the buyer can escalate the dispute and get one from eBay, who will re-charge you for it).
You will get a defect against your selling account if you don’t accept the return and the buyer escalates the dispute to ask eBay to step in, which they can do after 3 business days.
Once you get your item back, if you feel that your buyer has misused eBay’s Money Back Guarantee you can use the Report Buyer link next to the item in your My eBay. This likely won't affect the outcome of the return case though unless the buyer has very obviously damaged it.
