If you read paypal "Acceptable Use opolicy" it is very vague. Paragraph 2 section j reads

"""" (j) ammunition, firearms, or certain firearm parts or accessories, or (k) certain weapons or knives regulated under applicable law.""""

Here is the complete policy:

https://www.paypal.com/us/webapps/mpp/ua/acceptableuse-full?locale.x=en_US