The person filling out the form 4473 has to answer the first question, Are you purchasing the gun for yourself? That is all the seller really has to be concerned about.
The seller should not prevent anyone from helping make a choice on a gun, not the sellers job. If the seller has some reason of doubt that the purchaser is buying for another person, it is the seller's right to decline the sale. I have done that in the past, I turned away a husband and wife team. The husband had tried to purchase the day prior and when the background check declined him, he bluntly stated "Must be that exparte order against me". When he returned with his wife later and attempted to buy again, both were asked to leave.
I have only purchased one firearm at a big box store, and it was a nightmare of a transaction. It was at Cabela's and it was a rifle on sale. At any other gunshop, it would have been a 10 minute transaction with a quick check. Cabelas has some kind of goofy database they check first before they let you start the yellow form on an I Pad. Some chick with a name nothing like mine who lived in a different state with an address that matched my 3 digit street address but not the street name caused me to get flagged. After 20 minutes and a couple of managers involved to get over that, I filled out the yellow form 4473 on the Ipad and then went thru the standard NICS check. Then I waited in line to pay for it. Then I waited 10 minutes for the walk out. Over an hour to purchase a rifle and I was one of 2 customers at the gun counter. Never, ever, whatsoever, will I even think about buying a gun at any big box store unless they are selling it for a buck.