http://www.freebillofsaleform.org/gun.html
you need to have a form 4473 from atf if you buy a new gun. but if its used you should have them fill out something similar. at the least you want a bill of sale and write in the bill of sale that the owner knows that this gun isnt stolen or used in any crimes. that should protect you
if you want to avoid the atf dont buy and sell guns. thats what they do. yes you have the right to but… they have all this extra rules and regulations you have to follow if you want to be perfect.
also if your store buys and sells guns then some guy walks in with a gun you will not know his intentions until he says what they are. he will either say i want to sell/pawn this or he will say give me all your money. so if you dont trade guns you will avoid this mistake
also the serial # has more info than you would think. if it has been transported from state to state without a licenced ffl saying it was shipped legally it was involved in a crime. so a # could show if it was in a crime