If paypal knows you're buying or selling guns or gun parts, they will shut down your account immediately. Trust me....I have some direct experience with that.
ETA: I have no idea how paypal can allow gun parts purchases through ebay, but otherwise....they are banned. I guess because ebay owns them and they make more money from it.
I'm like you in that I don't understand why PP allows buying/selling of firearms parts through eBay, however, if I understand it correctly eBay and PP split ways a year or 2 ago. In fact, I got a message a few days ago from eBay announcing that they were going to be using a new pay system/company in the next year or 2. In fact, here is a quote from that message. "We have signed an agreement with Adyen, a leading global payments processor, to become our primary payments processing partner. PayPal, a long-time eBay partner, will be a payments option at checkout for eBay buyers."
I sell fishing lures on eBay and suspect that eBay will get more money from Adyen than PP on this deal.
Also-I see on Gunbroker where a number of Sellers accept PP. So, even though PP no doubt stops some exchanges I suspect that when one sells on a forum like this the likelihood of PP catching that it is a firearm is slim. But, that is just my opinion. If though, something goes wrong and there is a problem with the delivery and if it becomes exposed that it is a firearm in question, all bets are off.