Wireframe for Server-Side License Activation
Server-side app license code validation, as I imagine it, in a nutshell: If the token expires and there's no server connection, you have to figure out how punishing you want to be. I suggest you do not punish by default and assume people have good intentions. Possible escalations: Remote or server-side deactivation of licenses can be useful to prevent continued use after refunds.
With DevMate Closing Shop, Here's What You Can Do
DevMate are closing their doors. The announcement isn't news anymore, but since I didn't use their service, I didn't think much of it. I did report back in 2016 when they changed their pricing model. Nowadays I discover more and more people struggling to migrate away, and my blog pops up in their searches – apparently because I wrote a book on selling apps on FastSpring.