I think at one point several years ago you could get away with selling at least some of the Steam items, but that was a "loophole" that has since been closed. See this post from WGTNico:
https://www.wgt.com/forums/p/696334/4656685.aspx#4656685
While he mentions "stuff from bundles cannot be resold", I have been able to sell back some of the items I've received as part of a WGT bundle, so it may just be specific to Steam bundles.