Demand for Graniteware and enamelware in general varies depending on the are where you live. In an area with little demand you might be better off selling at an online site like eBay, etsy, or Ruby Lane in order to reach wide interest.
When looking for an auction house or dealer try someone who specializes in country, rustic, or farmhouse antiques or vintage pieces. Antique pieces mean pieces over 100 years old. Graniteware made before 1900 holds the highest value. Purple, brown, and green are the hardest to find colors so are priced higher than the ever popular blue or gray and white items.
To educate yourself more, head for a book:
"Collector's Encyclopedia of Granite Ware" by Helen Greguire
"Graniteware: Identification and Value Guide" by Fred and Rose Booher
The books will help you to learn the age, style, and type of items that you have. They can not be relied on for values as values change over time.