Here it is spelled out conversationally.
ATF Online - Training - Firearms - Federal Firearms Regulations Reference Guide Learning Theater - Straw Purchase Attempt Transcript
This is the relevent part:
You'd be breaking the law because to pass the background check you would affirm that you were the intended owner (purchaser) of the firearms, when in fact you were planning to FTF them to 4 other people
So it is illegal to buy a cheap gun if you just plan on selling it for a profit then?