I'm not going to quote morcey2's story that he is relating concerning what the cop told him as it makes the post too long. There has to be a lot more to it than just having a BOS to a prohibited person. If as stated the BOS may have had the question in it asking if the buyer was a prohibited person, and the buyer signed that he wasn't, how does that violate any existing federal law?
Taken from the Frequently Asked Questions to the ATF: A person may sell a firearm to an unlicensed resident of his State, if he does not know or have reasonable cause to believe the person is prohibited from receiving or possessing firearms under Federal law. A person may loan or rent a firearm to a resident of any State for temporary use for lawful sporting purposes, if he does not know or have reasonable cause to believe the person is prohibited from receiving or possessing firearms under Federal law. A person may sell or transfer a firearm to a licensee in any State. However, a firearm other than a curio or relic may not be transferred interstate to a licensed collector.
[18 U.S.C. 922(a)(3) and (5), 922(d), 27 CFR 478.29 and 478.30]
There is no requirement that a seller run a background check, only that you "KNOWINGLY" do not sell to a prohibited person. You are also required to not sell a handgun to a non-resident and that the person be 21 years of age. You would have to personally know that the buyer is a prohibited person. I would like to see a prosecutor that could convince a jury you knowingly, unknowingly (hope that makes sense) sold a hand gun to a prohibited person. A first year defense lawyer would have no problem beating that charge but I seriously doubt a district attorney would even let that case come to trial. I believe that the seller would have excellent grounds for a lawsuit against the government.
Now if this seller has a shady past and possible suspicion of selling guns to prohibited persons then I can see the police putting pressure on him hoping for a confession. Even with a shady past the prosecutor would still have to prove the seller knowingly sold to a prohibited person, but without a confession from the seller or the buyer, lotsa luck! Even if the BOS doesn't have the question about being a prohibited person, no federal law has been broken.