From what I understand, paypal also started canceling accounts when buying/selling firearms or parts from anywhere. I still have bought accessories from ebay (such as optics or lasers & such), but have been buying from gunbroker recently.
Don't forget, buying parts, minus the lower receiver on any firearm, can be shipped directly to you, even from the manufacturer.
I like gunbroker also because it does have those rare hard to find parts you can't find on any retail place.