Many people shop at swap meets to save money and get good deals, and cold winter days do not stop them.
Many people begin shopping more at indoor flea markets because they like the warmth, but the hardcore flea market shopper will still hit the outdoor flea markets even when it is snowing, because they know that since many people wimped out and went indoors, that leaves all the bargains for them. (Mile High Flea Market in Henderson, Colorado is open year round, yes, even when it is snowing, and the vendors and shoppers still come out like always.)
Sellers will still offer the usual swap meet fare, the used stuff and the new stuff, but no matter the time of year successful flea market vendors will still follow the rules: always sell what people want and need.
In winter people do not want swimsuits and shorts and sandals.
They want heavy coats with tall collars that can snap shut, furry hats that fold down to keep their ears warm and gloves to keep their hands and fingers warm.
They do not want sun tan lotion.
They want chemical hand warmers. They want thicker socks and warmer shoes and insulated boots. They want ski pants that the icy wind will not cut through. They want insulated bib overalls.
Air conditioners and box fans do not sell so well during winter, but electric space heaters do, as well as kerosene heaters, propane heaters, propane tanks, and propane hoses.
People will want snow shovels. They will want ice scrapers.
They will want to buy bags of rock salt and chemical melting compounds to spread on their sidewalks and steps.
People will also want snow tires. Used snow tires are great sellers, especially during the first few months of winter, but they do not move so well during the summer months. Most people do not think, or shop, ahead.
Used tire chains in good condition will also sell well, as will tow chains.
Cold remedies, cough syrups and cough drops always sell, but especially in winter.
Buyers will also be looking for snowboards, inner tubes and other things to have fun with on the slopes.
Many flea markets run their own concessions and will not allow anyone to compete with them. But many smaller markets do not have any concessions at all, and you can do quite well selling coffee and hot chocolate.
Just follow the rules and sell what people want and need. Change your inventory with the seasons and success will always be your friend.