If you made a PayPal account and then made the username and password publicly available, would they be able to stop you? This is the closest thing I came up with from their Acceptable Use Policy:

Can I add PayPal’s username and password fields to my website?

No. A customer’s PayPal username and password are sensitive information belonging to the customer and safeguarded by PayPal. Many customers use PayPal because they do not feel comfortable entering credit card information directly on merchants’ websites. Although your website is secure and trustworthy, PayPal must prompt customers for usernames and passwords on a PayPal-hosted page to assure them that PayPal is safeguarding their personal financial information.

I’m not talking about being a company trying to sell stuff through PayPal. I’m talking about creating a PayPal account where you list publicly your username and password. It doesn’t seem like it would be against their TOS but I can’t tell.