By default we do not let publishers sell only keys to other stores. We request that they upload a file in order to have their product be purchasable. You can read more in our quality guidelines: https://itch.io/docs/creators/quality-guidelines#avoid-only-uploading-keys-or-li...
We do make exceptions for some sellers, but there has to be a reason why they can not provide a build of their game on itch.io.
Thanks