Other
End User License Checker
MC License offers a publicly available end user license checker: https://mclicense.org/check
When a key is searched, it will return its validity status, expiration date and active server count.
For security reasons, it does not provide additional data which the plugin author has access to, such as the plugin itself or the user field.
Please feel free to share this page with your buyers so that they need not bug you to retrieve information about their license.