From Guns.com:

When it came to retail sources such as gun shows, flea markets, firearm stores, and pawn shops, only about 10 percent said they were able to obtain their weapons from such outlets through purchases or trades. Of those, the majority reported that a background check was conducted as part of the sale, although in many cases they did not purchase it under their own name.
In all, only about 1 percent of prisoners who used a firearm during their crime had obtained it through a retail sale.