http://dailycaller.com/2013/02/11/where-criminals-get-their-guns/
Look at those stats, from a poll of
convicts, who know where they got their guns better than anyone else.
If you don't believe convicts themselves, try this:
http://www.pbs.org/wgbh/pages/frontline/shows/guns/procon/guns.html
It's from PBS, but think about what it's saying... criminals get their guns primarily from a few sources:
Straw sales (with the gun dealer unaware that it's a straw purchase). There's no way to prevent that, if the criminal has a friend who has a clean record.
Shady dealings by FFLs. Again, background checks have no effect, because FFLs are already required to conduct background checks (or equivalent) prior to transfers.
Other methods of acquiring firearms include obtaining them from friends, or stealing them.
In that liberal (PBS) reporting of an ATF-sponsored report, there is not a single mention of private transfers, as in unsuspecting upstanding citizens selling to criminals.
Personally, I've never sold any firearms privately, but if I did I'd ask the buyer to bring one or two other guns, to demonstrate that they were able to acquire guns before, and that I'm not selling to someone who is desperate for a single gun with which to commit a crime. It seems like a good proxy for a background check to me. It avoids the hassle of a background check, and if the buyer ended up committing a crime with gun I sold him or her, at least I have the peace of mind that the buyer had other guns they could have committed the crime with, making my karmic (not legal) responsibility essentially nil.