Advertisments, forced into my feed without my consent are certainly undesirable, but the PubSub nature of Nostr would seem to naturally support an exchange of "Offers to Sell" and "Offers to Buy." Given a sufficiently robust ability to REQ, I should be able to subscribe to receive "Offers to Sell" for some specific item or service, potentially with some set of characteristics determined by me. For instance, I'd like to be able to publish an "Offer-To-Buy," or subscribe to receive "Offers-To-Sell, for a "Model X SmartPhone with XXX memory for less than $XXX, delivered to my address in Zip Code before XX/XX/XXXX." Ideally, I would only see offers which did, in fact, match my Offer-To-Buy or my subscrption to Offers-To-Sell. A similar pattern could support a wide variety of exchanges.

This isn't quite "advertising," but it would be a means to accomplish at least some of what is the purpose of advertising: matching potential buyers with potential sellers.