- You will need permission/license from the other
contributors to sell proprietary licenses.
- You need a scheme that fairly distributes the
licensing revenue so that it motivates people to
contribute. You could pay out shares of the revenue to
them, or you could make it a public service effort by
donating a share or all of the proceeds to a organisation
with aims that benefit the general public, for example
FSFE.
A Contributor License Agreement (CLA) can be
used to set up such a model. Usually, FSFE would not
recommend using CLAs that enable proprietary licensing.
However in your situation this is already the case, so in my
understanding it would make your software more free (because
others can contribute to it). Please be aware that this is
not an official FSFE position.