Federal firearms dealers are required by federal law to do background checks on all gun sales.  Casual gun sales at gun shows and other venues, as well as online, do not require background checks unless the seller is a licensed firearms dealer, and while organizers of Vermont’s gun shows required background check this past year, the law does not require it.  It has been estimated that nearly 40% of all gun sales nationally are completed without a background check.

Source:

http://www.nytimes.com/2013/04/17/us/seeking-gun-or-selling-one-web-is-a-land-of-few-rules.html?_r=0