Purchasing a gun with 15 round mags

What is the legality of purchasing a gun that only comes with 15 round mags but not accepting the mags or having them sent back? I talked to a ffl about it and they said they would just send them back and it would be okay but is there some kind of weird legal technicality where I could get fucked up since I would be buying the gun on the internet and having it shipped to the ffl?