Actually that’s not strictly true. ALL FFL dealers require ID, and private sales are subject to enough laws that you really have to at least verify the buyer is:

A) 18-21 depending on hand/long gun,

And

B) In state, as selling to someone out of state is mostly illegal (depends on your state, but it’s the safe bet).

Most gun shows are stocked by FFL dealers but even if not, the smart private sellers who don’t enjoy prison still ID (and write your ID# down on the bill of sale at that in case you are a fuckhead and cops come sniffing later, it’s mostly to CYA, but most people CYA.)