This article from Roll Call states:
The plan is expected to stop short of language currently in the bill that would require background checks on nearly all gun sales, including between private parties. Instead, Toomey aides said, the proposal would require background checks for private sales at gun shows and on the Internet, two areas that are currently exempt.
Internet gun sales require a background check. You can purchase a gun at any number of auction sites (not EBAY, btw) and the seller will ship ONLY to your FFL, a federally licensed gun dealer, who will perform a check and fulfill anyother local requirements. Furthermore, private citizens can not ship a gun in the mail to another private citizen. Mailing a gun must go to/from an FFL. FFL’s are HIGHLY regulated by the federal government and undergo frequent and random audits. ATF grants these licenses and you bet they are closely watched.
The background check is conducted at the local FFL because local and state requirements differ greatly and the most efficient way is to have the local FFL handle those requirements. If you try and buy an AR-15 or AK-47 and you want it shipped to CA, no one will sell you the gun on an auction site.