What’s the license for these assets?
They actually have fairly simple classifications, I recommend you give it a look when you've got the time. Just about any dev with sense is gonna ask for that info because they can run into real legal trouble if they use something that turns out to have a restrictive license attached.
You don't have to do anything special, like register your work or anything. You just have to detail the license in like a document in the pack or on the store page (preferably both) and that's enough to cover it legally. It's basically just a pre-prepared contract you can copy/paste.
Here's a good example, the commonly used MIT "open source" license for software: https://opensource.org/license/mit
Just find a license that's relevant to your work and needs, so basically something in art or software technology.