Well, AR pistol kits are legal now. So the seller shoudn't give you a bad time.
Now, depending on who you buy the lower from (say online), they may or may not invice it as a pistol. Some will, some won't. Doesn't hurt to ask if they will list it as a pistol for you to your transfer FFL. The transfer FFL has to list on the 4473 "pistol-stripped lower". That's waht I did for mine. And of coarse, if your state has any "handgun" rules/forms, follow/fill those out too.
The only requirment for making a pistol, so the word goes, is a receiver/frame that has never had a stock attached to it. IF it has a stock on it first, then it's a rilfle. The stripped lowers are recorded as stripped from the factory.
You can make it either. If it is papered a pistol, you can coverted it to a "rifle" and back to a pistol again. Just make sure that the short barrel (under 16") IS NOT ON THE LOWER when you attatched the stock.... or you will have a stocked pistol, which is a NO NO w/out the right federal paper work.
Depending on your *insecurity, paranoya*, I suggest having a copy of the 4473, w/ you, IF the dealer will allow you to get a copy. That way, no one will claim you have a rifle w/ a short barrel. I plan to get one of those pistol grip plugs from Brownells....and put a copy of my invocie and state paper in there showing it is a pistol. Just in case (call me paranoid). W/ the ban just newly expired, some people might assume it is still illegal. That is just me. Better safe then sorry.