Speaking as a vendor, I can assure you, if it was a "lawyer" issue, they wouldn't be selling anything because there's just as much liability exposure no matter if you sell a belt-fed class III or a blender ... it all depends on who wants to push it and how far they want to push it. For that matter, I can go to your garage sale, buy some piece of crap I haggled you down from fifty cents to a quarter then sue you over it and it'll cost you hundreds, sometimes thousands just to defend yourself. Matter of fact, in most states I don't even have to buy an item from you, I can steal it and still sue your butt off.
The issue comes in dealing with the BATFE and trying to avoid creating problems for themselves. It doesn't matter what the law says, it matter how much of a prick the g-man/g-woman is. ML's don't require NCIC but since it's inventory in/out, subject to sales tax and so forth, a store owner doesn't want to loose money trying to explain why a "gun" was sold without a NCIC being done - worse yet, it could end up being a trip to the pokey followed by a court case even if no wrong was done .... that all costs time and money.
If I were you and you wanted the gun and the price was right, what's the big deal in doing the NCIC?
Now, if it's an issue with moron/jerk clerk, then you walk out like I did at Johnstone Supply. I've been in the industrial business for 17 years and did tens of thousands of dollars with JS but the worthless piece of crap they hired as a store manager doesn't take any of that into account while he ran his mouth about "Florida law" yet he does not know the law because the more he ran his mouth like a big shot, the more his stupidity showed. I tried to follow that incident up with corporate but they could care less so I took my business to a couple family-owned suppliers, both with awesome customer service and far better prices too - of course when I do consulting work, I make sure the business goes anywhere except Johnstone. Perhaps it's small potatoes to them but one moron employee cost the company about $300k in sales over the last three years.