It is a good rule of thumb to get a photocopy of the sellers Drivers License, that way you are sure you have all of the information you need in a legible form. All you need is the seller address though.
Here is a link to the November FFL Newsletter and on the top of page 7 it covers this exact question...
"...a nonlicensee may directly ship a firearm to a licensed importer, licensed manufacturer, or licensed dealer for any lawful purpose. As such, the nonlicensee may arrange to ship the firearm directly to a licensed dealer in the State where the other nonlicensee lives. The receiving licensee is required to enter the name and address of the nonlicensee from whom the firearm was received. A copy of the nonlicensee’s driver’s license may
be used to verify this information, but there is no regulatory requirement that it be provided by the nonlicensee."
Some states (NY, CA, possibly others) have laws that prohibit any non-licensee from shipping guns to anyone inside of their state. CA goes even further... Also some larger chain stores (Bass Pro for example) have policies against receiving guns from anyone other than an FFL holder.
Hope this helps...