The only thing the Nostr protocol passes around is text. Clients would have to cooperate not to show what the text links to, unless the user paid. Probably the better way would be for you to link to a sample, and provide a second link to a site you control for the full product. That site could enforce payment.
Discussion
I think this could be implemented by the client like iris, amethist, etc. They even could take a cut of the payement. I am happy to pay 1% fee.
I asked @jack to sponser it. Did I ask the right person?
Well, he is probably the wealthiest person on Nostr, and is interested in its success.
On second thought, you’re right and I wasn’t being imaginative. Nostr clients could accept delivery of a key (through DM?), and send that key to request specific content. The key could be sent by the content provider in response to a lightning payment from the client.