as far as i know, the laws only apply to ownership.
The laws only apply to the TRANSFER of the firearm through an FFL. Fed law only restricts pistol ownership of minors. Any adult 18 or older can possess a handgun. (look at my sig for the legal site)
Back to the OP. Legally you would be clear to buy it but not transfer it until you're 21. The only problem would be if the dealer or seller has an issue with it. Definitely talk to the dealer first. If you can't find a dealer to wait the month, there's no reason to ask the seller. You may have a better chance getting the dealer to agree if you're on good terms with the shop and have made other purchases there before (long guns, ammo, or other things that they sell).
Personally I wouldn't even bother the seller with the info that you are under 21. In my view, he really doesn't need to know. All he should need to do is accept the payment and ship the gun (as long as everything is legal).
***This advice is definitely not going to win me points here, but it is what I would personally do. I've just run into too much **** from people who think they "know the law" better than I do and have become rather jaded about the whole process. You can only explain the legalities to people so many times before you just decide its not worth it unless they ask. (I've only got 4 months until I turn 21 myself, and I've bought and/or sold 5 or 6 handguns since turning 18). You may have a different opinion and that is fine. Please don't take my advice if you have any moral or ethical qualms about it.
NICS won't pre-qualify you as I'm pretty sure that you need to have a 4473 to correspond with each call (but I'm not a FFL so it is very likely that I'm mistaken). As long as you have passed the check before, you'll be fine (baring any felonies that you've been convicted of since your last check
). The only difference between the rifle check and the pistol check is that you have to be a different age, nothing else is changed.