FIREARM PARTS AND ACCESSORIESThe reason for the tight rules is obvious: PayPal and eBay are trying to stay in the good graces of the liberal state such as Illinois and California, which have very tight rules about "assault weapons" (ugly guns) and "high capacity magazines" (>10 rounds).
You may not use PayPal to purchase or sell any firearm receivers or frames, components and parts of receivers and frames, or "cut" or "80%" receivers. PayPal also does not permit assault weapon-related parts and accessories, firearm silencers, and kits designed to convert a firearm to have automatic firing capability. This includes the sale of any parts or accessories prohibited for sale by the National Firearms Act or other federal or state law, including items related to short-barreled shotguns or short-barreled rifles, fully automatic weapons, large-capacity magazines, multi-burst trigger activators and camouflaging firearm containers.
For more information about the National Firearms Act (NFA) and its requirements, please refer to the Bureau of Alcohol, Tobacco, and Firearms website.
You may not use PayPal to buy or sell high-capacity magazines (those which hold more than 10 rounds), ammunition with propellant (e.g., gunpowder), or blank ammunition. The term "ammunition" means ammunition or cartridge cases, primers, bullets, or propellant powder designed for use in any firearm.
There are stories floating around on the web about people whose PayPal accounts were summarily closed after they had a transaction where certain keywords relating to firearms had appeared.
I myself have used PayPal to buy firearm related stuff on eBay and other sites; I guess I will have to be more careful.