You can sell your licenses of Eventide plug-ins.  I believe there is a $25 license transfer fee imposed by PACE, but you are able to sell any Eventide plug-in license for whatever price you'd like.

However, each license has two activations.  You may sell a license, but both activations will go with it.  If you genuinely have two licenses then you may sell one and keep one, but if you only have one license, you may not sell one of the activations.

Dan