Nothing. As long as the seller and buyer live in the same state and their state has no law prohibiting such a transaction.
If the seller and buyer live in different states they BOTH are committing a Federal crime.
Federal law also allows the return of a a repaired firearm directly to the owner, no FFL needed.
Need a FFL in north Dallas/Plano/Allen/Frisco/McKinney ? Just EMAIL me.
$20 transfers ($10 for CHL, active military,police,fire or schoolteachers)