[grabeh]

The rights Grammarly (https://www.grammarly.com/terms) take to use your data is limited as follows:

"in connection with the provision of the Software and the Services and to improve the algorithms underlying the Software and the Services."

This on the face of it wouldn't extend to selling your content. Obviously terms of use can change, but a material change like granting Grammarly the right to sell content would usually oblige Grammarly to notify users and at that point users could take a decision to stop using the service.

Also, realistically any proposal to sell data would make use of the service for great swathes of enterprise users a complete non-starter.