Your buddy is mistaken. It's true that in general, it's legal for a private individual to rent a booth at a gun show to sell his collection, or for private individuals to sell to each other, without a federal license. However, if you don't have one and make a habit of buying guns in order to resell them, you'll be in a peck of trouble once the authorities notice. See the current thread Virginia Woman Charged with Dealing w/o a License
for an example...
"Once the writer in every individual comes to life (and that time is not far off), we are in for an age of universal deafness and lack of understanding."
(Milan Kundera, Book of Laughter and Forgetting, 1980)