I've never understood this practice, Why sell a firearm until you know the person is legit ?
I've known a couple of shops to do this. People will use the shop as a free backbround-check service. They'll take up a great deal of the shop's time, then walk out if they get delayed or denied.
In that case, the proprietor can issue a refund if the purchase isn't completed, but retain a fee for his time and efforts.