It is allowed to re-sell Matomo (Piwik) and provide it for free, or for a fee.
If you are providing a service reusing Matomo (Piwik), you must also reuse the word Matomo with care. Please see the Matomo Trademark page for more details about how to reuse the name Matomo in your modified version of Matomo. In particular, it is not allowed to use Matomo in the URL domain name.
We would love to know if you create something interesting using Matomo: drop us a message.
We also recommend that you partner with us: we provide an amazing Matomo Cloud (formerly Piwik Cloud) service which you can resell to your customers.