Anything involved in the "firing of a gun" is banned. That means bullets, brass, barrel, trigger, magazines etc. You can still buy and sell gunstocks and most reloading tools.
See Daily Bulletin, July 31, 2007:
On July 30th, internet auction site eBay announced it will change its current policies regarding sales/auctions of gun parts and reloading components. Currently, one cannot sell firearms on eBay, nor live ammunition, nor a variety of “black-listed†gun accessories such as high-capacity magazines. Now, eBay will prohibit the sale of “any firearm part that is required for the firing of a gun.†This will include brass, bullets, and barrels. The new policy was announced by Matt Halprin, eBay Vice President:
“In mid-August, we will be updating our Firearms, Weapons and Knives Policy to place more restrictions around gun-related items. Once these changes take effect, we will prohibit listings of any firearm part that is required for the firing of a gun. This includes items like bullet tips [sic], brass casings and shells, barrels, slides, cylinders, magazines, firing pins, trigger assemblies, etc. Please read the Firearms, Weapons and Knives Policy for more details on our current policy."