I received the task to build the website of a little company and they wanted me to build it with concrete5. They have also chosen and purchased a theme, but only one licence.
Since I want to build the website on my local machine first, I wanted to ask if there is a possibility to migrate websites with purchased themes once I finished building it on the local machine, even if we have only one licence (since the theme would then basically bee used twice ; also when purchasing a theme, they ask to assign the licence a project, making it only available for this one).
This is totally fine, the license is for one site, not one install. So staging environments are totally OK. You might have to get your client to add your URL for automatic updates, but I'm not sure on the process for that. You can just FTP or rsync the files down and install locally, though.
Really, all that the actual mechanism behind checking the licenses does is to show you when there are updates, which, if you're going to be customizing the theme at all, you might not even want. Otherwise, the client could see there's an update and blow away your customizations. So in that case, you might not even want to associate the license with the marketplace at all, instead just leave it unassigned and download a copy of the files manually.