The API usage guidelines are very simple.
1) Significant API users should ideally provide a user agent, so that they can be identified in analytics and in case of a potential error. If a user agent is not provided and there's a problem with an API user, then we would have no choice other than blocking by IP, and that would be very unfortunate for all involved.
2) If an application serves as a client for Repository Manager repositories, or the Open Shop Channel repository, then visible credit to Repository Manager, Open Shop Channel and dhtdht020 would be very very appreciated. It's difficult work!
It would also be admired if terminology like "Open Shop Channel client" would be used to describe the software.
3) Have fun coding! =D
The API should NEVER be used for the sole intention of replicating's online library with the official repository under a different name or website, and should also never be monetized by web advertisements or by direct purchase. The APIs for official instances of Repository Manager is a service to the community and is intended to enable development of helpful utilities and clients, as well as intended to provide valuable data and capabilities to application authors.
Repository Manager! So sophisticated!