@kbar Hey Kent, thanks for the tips, some great information there.
Good timing as well, I just managed to interface with Gumroad and test it a first license key.
I used "requests" module for Python, even though I'm not sure it's the best fit, since it requires 4 other libraries to be copied with it.
Maybe pycurl is better for this?
There's also pygumroad but I'm not sure if it will work.
About the offline key, could you elaborate how it's being used?
Edit: Nevermind, I think I got how you meant it. The gumroad confirmation allows the creation of the key.
Having your own database is probably the best, but developing an AWS Database for my first little plugin seems a bit disproportional. Actually I was interested if there was a seller like Gumroad for C4D Plugins that would take care of the licensing for me, but I don't think there is something like that. I was surprised by the learning gap between developing a plugin and protecting it, and wondered how others are handling this. But other than bigger companies like Redshift and Insydium, I can't think of many "indie" protected plugins, so I'm afraid I agree there's probably not a big market for it. But it's so much effort to get there, no wonder there aren't many people developing. It's kind of a feedback loop.