From what I've seen, Unity Packages have no native DRM support.
If you sell items on the asset store, there's nothing that can stop a customer from redistributing those items to non-customers.
The typical way to fight this is to keep updating the asset with new and improved features. This should happen organically if one keeps supporting the product they are selling.
Ideally all game developers are professionals and believe in rewarding or reimbursing the work that helps them.