Online Indie Bookstores are either like marketplaces but in miniature, in that they sell books directly, or they list they books, but direct readers to a larger marketplace to make the purchase. Either way, they have only a fraction of the books, and a fraction of the readership, of a larger marketplace such as Amazon or Barnes&Noble.
Positives: Online Indie Bookstores are often set up with the intention of supporting Indie authors, and some readers may prefer to shop at these stores.
Negatives: These websites may get very little traffic from genuine readers looking for books. The websites often have no quality standards, which could negatively impact the reputation of your book, and decrease the likelihood of genuine readers making a purchase. Some websites charge money in exchange for a listing, in which case you may not see a positive return on that investment.