Are users permitted to sell the models they create using this app?
Create your own 3D models and 2D sprites! · By
Yes, please refer to the included readme.txt file which states:
All included blocks are 'Creative Commons Zero' licensed (see below).
Creations made in Asset Forge can be used in any sort of production such as personal, education or commercial.
Block/model license (Creative Commons Zero, CC0)
http://creativecommons.org/publicdomain/zero/1.0/