Same happened to me today. In at 10:55, out at 11:15. Some shoes are easier to authenticate I guess. I got foams. You can spot a fake one a mile away so I guess they pass through pretty quick.
DO NOT refund the money. You won't get into any trouble since you made an honest sale. Just ignore the guy. He bought shoes from you in person, which means he got to inspect them up close and made a conscious decision to purchase. I assume he paid in cash right? He has no recourse and is likely...
In total, I've sold 3 pairs and purchased 5 pairs from stockx. Ive been very happy with everything so far. My 5th pair is actually still on the way to them for authentication, but I'm hoping it will go as well as my other transactions.