In this area, New England, a Co-op designates a (usually) large shop where the owner/manager rents out spaces by the month. You can usually just rent shelves, or whole spaces big enough for furniture, etc. Some require you work part-time but this is usually in return for free/reduced fees for your space. I love co-ops because you can usually find different dealers specializing in different areas of interest.
A co-op is a business owned by it’s members. I would say to sell there you have to pay the dues, work some times and like a club different people have different jobs. I was in a Christmas Craft Co-op for years. We opened a store front each year, and for years it worked pretty good till we got a gal doing bookwork that thought she was the boss. The co-op keep losing crafters (including me) till it got to the point there was not enough left to make it work.