I've been there; and, from talking to the folks selling things, one just shows up and sets up. At some point, a person will be around to collect the fee. They said it was $10 for the day.
That sounds strange. That's Calvert County/Fair Board property. Is it a County employee that collects the $10?
If I know the County, and I do, they wouldn't allow anyone to set up or pay a fee without a list of vendors. The Commissioners and the Departments are real risk averse.
Not sure of the relationship between Cal. Co. Gov. and Calvert County Fair Board. It may even be a lease type thing where Cal. Co. Gov leases the property to the Calvert County Fair Board. I dont thinke there is even a link from the Cal. Co. Gov website to the Calvert County Fair Board website.
Calvert County Fair
On a post in the area where the flea market/yard sales take place, there is a sign with a telephone number on it for more information. Or at least there used to be last time I was over there.