My understanding is that you cannot call in a background check until you have actually completed a transaction. That generally eliminates the folks who just want to know if they can pass it.
I let folks know up front that they may not get an answer for up to 3 days and tell them in advance that I will NOT refund their money unless they do not pass the background check. Then, if and when they fail, they will receive all of their money back except for $5 which is what I charge for the background check.
I just went thru this about a week ago and thought I was gonna have to call the cops on a guy when he was "delayed," he WANTED a refund, and he wanted it NOW, but he settled down as I reached for the phone. He ended up failing the background check and then the FBI wanted me to hold off on letting him know, til the local cops had a chance to see if they wanted to do a sting and pick him up at my shop. I called the buyer back immediately (as I had told him I would), he was there in 10 minutes, and I gave him his money back (minus the $5) REAL FAST!!!!! Like I wanted to be involved in having a guy arrested in my store!
I'd prefer NOT to get shot trying to get some low-life arrested.....
Let the customer know up front, and you'll make your life SO much easier!