It is possible to be a retailer, and a monopoly, without being a monopoly in retail. Indeed, most determinations of monopoly power use a much narrower definition of the field they hold that power in.
For instance, Amazon holds a significant percentage of online book sales—and, in fact, of online retail sales in general; see one of the sibling comments for references.
For instance, Amazon holds a significant percentage of online book sales—and, in fact, of online retail sales in general; see one of the sibling comments for references.