Part of it is the fact they are in California, so you can blame THAT state more than Paypal or Ebay itself.
I went through a long email conversation with Ebay about this, and they take the tact that if its illegal to sell in any state, IE, High cap mags, in multiple states, Machine gun parts in Washington State, IE, a parts kit from an AK, that they could be held responsible for facilitating the breaking of another states, or California's laws.
I asked them then why can I buy a bong, or a switchblade? They told me that only CA legal switchblades can be sold, with a 2" or lesser length blade, and that if it appears a bong is sold for drug use, they pull the ad. A bong sold for use with tobacco, is legal.
Now what I have done is to accept credit cards, but with a 1% less markup, than paypal, and you know how many people take that 1%, not very many.
Its convenience, I tell users to never mention the actual item, IE, magazines, AK parts, etc, in the payment notes, refer to the item number, thats fine, because chances are if a keyword isnt in the transaction, they probably dont take time to check.
I too like breaking their rules, and still using the service, but too many people just will not use anything else, so until I do get thrown off, I am flaunting their laws as much as I need to.
Bottom line, if the pro gun people got off their asses and helped change STATE laws, you could make it very difficult for ebay and paypal to have an easy out selling these items, because if it wasnt illegal, Im sure they would be happy to take your money.