> The only sale/transfer allowed without a NICS check is between two residents of the same state when the seller has no reason to believe or suspect that the purchaser is prohibited.

Say the seller transfers a gun they've owned for several years to an out-of-state prohibited person. The gun turns up in crime. The original purchases lies and says they sold it just one year after ownership to an in-state person, and haven't possessed this gun for several years. There's no requirement for sellers to keep record of who they sold firearms to, so it's hard to prove this defense wrong.