I believe that as long as the material is within the scope of Time Travel, then yeah, we should allow them to post.
I don't necessarily think that mugs, T-Shirts and the like should be allowed to be promoted.
However, it seems to me that published authors writing on the subject of Time Travel should be allowed to post.
Being a fan of time travel am always on the lookout for new books to read regarding Time Travel. Having the authors themselves posting here makes looking for new books to read all the easier.
There are numerous brilliant Sci-Fi authors out there. It seems to me that it would be awesome to have published Sci-Fi authors posting on TTI and we should encourage the authors that do promote their books on the TTI site to participate in the Time Travel discussions.