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 Dallas/Plano/Allen/Frisco/McKinney ? Just EMAIL me. $20 transfers ($10 for CHL, active military,police,fire or schoolteachers)
Plano, Texas...........the Gun Nut Capitol of Gun Culture, USA https://www.youtube.com/watch?v=pELwCqz2JfE