@J12t I think that it may be reasonable to insist that ads must be instance and client local. They may be inserted into an Inbox or timeline by local software, but should not be inserted into any Outbox other than that of the advertiser. This rule would make ads effectively opt-in.

Users should be able to subscribe to feeds containing ads and offers. For instance, I am moving to a new town and may wish to temporarily receive move-related offers.

Ad-only feeds need a bot-like profile flag.