let the buyer know that you only have a 30 day warranty ( MUST BE SPECIFIED IN LISTING ) and that the 30 days has passed. If they file a claim with az then you can simply show amazon your 30 day return policy from the listing.
If the listing doesnt specify then you may be in for an a-z claim that you will probably lose.
If the item is cheap i say just accept the return to get a probable angry customer off your back.
A lot also depends on what you are selling OP :( yes it sucks - between the 30 day "policy" (which can be abused) and the 180 day chargeback provided by CC companies...it's a very tough selling situation
The good news is, if you have a quality product that is authentic, these situations are rare. Yes, you run across the occasional scam artist and other things can also go wrong, but this should be a tiny percentage of your overall sales.
There is no way to eliminate risk, you can only manage it.