[Canada]

I know partnerships and account selling/trading/buying is disallowed but I was wondering if exchanging feedback is considered to be black hat activity as well?

['Snorlax]

From reading the Help Docs and not seeing anything about it, I would say this is allowed. You're not exchanging account details or anything like that, so it won't go against the rules.

[Krew]

Not allowed.
Source: I received a warning for it.

[Emylbus]

Not allowed.
Source: I received a warning for it.

Correct, it is not allowed. It is considered an ebay partnership, forbidden by the Disallowed Blackhat Activity Help Document.