Things you create with Spine are your IP, you own them and you can do what you like, sell them, etc. Those things include your Spine project files and things you export: JSON and binary data, images, texture atlases, and video.
Note that creating derivative works of the official Spine Runtimes (which load and render the JSON and binary data you've exported from Spine) is granted by licensing Spine. So, while you own the JSON and binary data and can sell it, anyone who wants to use the official Spine Runtimes still needs a Spine license.
The Spine Runtimes are not mandatory, anyone could write their own code to read the data exported from Spine and do whatever they like with it, as long as they don't use any code from the Spine Runtimes. That is a whole lot of work though and most Spine users use the official Spine Runtimes. We put a lot of effort into making them good, easy to use, and keep them up to date with new editor features.
To distribute the Spine Runtimes you need to include the Spine Runtimes license. The Unity Asset Store does not allow you to include any other license besides their own, so unfortunately you can't distribute the Spine Runtimes via the Unity Asset Store. You can distribute your own files and you can give instructions on how to get the Spine Runtimes, but you can't include the Spine Runtimes in the files you put in your Unity Asset Store listing.
