I have a few questions.
1. Are you asking this question prophylactically or have you sold an item that the buyer thinks is fake?
2. If the latter, has the buyer filed a dispute?
3. Does the buyer want to keep the item if it's proven to be authentic?
4. Or does the buyer want to return anyway?
If #4, I would just take the return, get an authentication and include a statement when you relist. Potential buyers can contact authentication service to confirm that they did that item. (In fact, I did that very thing when I sold a Burb item. Buyer claimed she took it to a Burb store and was told they "couldn't determine authenticity," a typical response since they don't authenticate. I got it back, had it authenticated by A4U and relisted it.)
If #2, yes, A4U provides documentation acceptable to ebay, PP and c.c. companies.
If #3, spend $7.50 for an authentication and forward it to the buyer for reassurance.