I've spent over 20 years thinking about it, and not been able to figure it out.
Since it's clear to you, perhaps you can clarify how the Agile Manifesto applies to software product sales when there's a single developer and once-per-year product cycles?
Assuming your definition makes sense, what are the "interactions ... processes and tools" between me-the-vendor and me-the-developer?
What is the comprehensive documentation that I'm supposed to avoid?
What is the contract negotiation?