"To sell the category" means to promote or market a specific type of product or service in order to increase sales and interest in that particular category. Full definition
The challenge is trying to figure out what category is indeed a best selling category as there are a mix of categories that had daily sales and other categories that are missing daily sales. (betterbooktools.com)
The students were really well prepared with questions about content, editing, formats, revenue, distribution and top selling categories. (lulu.com)
After all, romance is the single biggest selling category of fiction of any type. (publishingperspectives.com)