Other electronic payment systems, I don't know, but guns and certain gun parts can't be bought or sold via Paypal.

how about sellers that demand postal MO when selling high end firearms that are worth thousands? Do these people not know that USPS puts a $1000 cap on these things? That means you gotta "stack" multiple MO's, not only it's a major hassle, you also have to pay the same $2 for each MO, plus stacking MO is a great way to get IRS's attention.