That is Bloomberg's justification in suing the gun dealers. His point was that even though the buyer provided all the documentaion the dealers should have read thier minds and known that it was a straw purchase therefore not sold the guns.
You are orrect that a dealer doesn't have to sell if it doesn't feel right according to the ATF. Then the ACLU and civil rights groups get involved.
In this situation (some have other opinions) as either buyer or seller I would get the contact information for the other party in case the ATF ever comes looking for that particular firearm (it was used in a crime, etc.) you can give them the information on where you got it or to whom you sold it.
Or better yet, as I previously said, if you don't know the person, just don't participate in the transaction. I don't care how awesome the firearm is, if you don't know them and they seem suspicious, it simply isn't worth the trouble.