the art is under the creative commons license which means if a single artist denies your right to sell, then it's illegal.
All the Trem 1.1 media content is under the commercial creative commons license, which means all the artists have already agreed that it's alright.
User made maps, on the other hand, have their own licenses, so if a person wanted to sell a bundle of Trem plus user made maps, then they would need permission from all the individual artists. Otherwise, you're free to sell Trem it as you want, as long as the source is included.