(back to on-topic)
I can't think of a review site like you've described, although Bizrate.com might be close. They have ratings for online stores, but it's more a traditional 'shopping for stuff' site. I do think their ratings are used by other shopping sites, though, so it is worth checking out.
If you wanted to try an unorthodox approach, though, you could look at setting up more of a Facebook presence. I'm not a big fan of their whole 'we hate privacy' ethos, but there are a lot of people on there, more are joining, and it probably has a pretty good overlap with the webcomics-reader demographic. I've posted links on there to Sheldon before, but maybe a Sheldon widget/application would drive some more traffic to the Sheldon store.