omiI was listening to another interview with Brian Bendis, writer for several Marvel Comics titles like New Avengers and Ultimate Spider-Man. Anyway, one point I found interesting was that he mentioned that aside from best-selling authors like Stephen King, comics does sell, and sometimes more than most books (he was citing Hillary Clinton's book, which sold around 11,000 copies).
Of course I'd like to clarify that the mainstream comic companies distribute through Diamond, so most of what they release do sell-out whether the market buys all of them or not. Second-printings are the exception and I expect that Diamond must have exhausted their comic stocks for a specific title to warrant a second printing.
I think an important part of comic sales is the fact that they're reaching more audiences nowadays, especially since they're making appearances in bookstores. The one qualifier, however, is that the comics must be compiled, usually in the graphic novel format (and for similar reasons, manga titles sell quite well and popular in bookstores). Locally, the same is true. Independent comics find it difficult to land in bookstores but those that are compiled in trades (Zsa Zsa Zatturnah, After Eden, Andong Agimat) do manage to make it to comic book shelves, assuming the publisher makes the effort to distribute them there and willing to shoulder the (expensive) consignment fees. Fully Booked is an exception as they do stock comic singles but I see that as a reflection of the business owner (he's a comic fan) rather than standard bookstore business policies.
While some people in the comic community might shy away from the term "graphic novel" (it's still a comic anyway, no matter what term you use), I think this marketing tactic has paid off and graphic novels are slowly being more accepted as "books" in various industries. I even find it interesting when comics do make it into books, such as one of the stories in China Mieville's collection of short stories, Looking for Jake and Other Stories which is a full-blown comic.