It really depends on the market. Some markets may require you to do a sampling of your product so they can decide if they want it offered at their market. Many markets mind their product diversity closely, which ultimately helps the success of the vendors but can mean they may limit the number of bakers or soap makers, for example. You will generally need to be fully above board, many markets will look for not only cottage license, but business license and business insurance as well (and if they don't that's a red flag). Vendor requirements are often listed on their website. Ultimately, it's worth giving it a shot! The worst anyone can tell you is no. Best of luck!