Since sales are in dollar and the OP does not specify a jurisdiction, I will assume Washington state US. You may need a business license, depending on the municipality. There are various permits required for sale of certain wares, such as food, explosives or poisons, but otherwise no special municipal permits are required. A state money transmitter license is required, and there are a number of bond and auditing requirements that also have to be obeyed. The state has a website which implies the pertinent restrictions, but does not actually explain them or how to comply: you would need to hire a lawyer to determine what you have to to to be legal.
Those restrictions are not limited to flea markets, they regulate all bitcoin sales in the state, including internet and yard sale.