Think of all the mascots that pop up every year during the sports seasons, both college and professional -- big doofy grins, enthusiastic waves, and bright colors attract the attention of fans (whether they want to stare at a big blue dog or not). You can apply this principle of attention-grabbing to your flea market stand; find or create yourself a mascot, and you'll inevitably draw people over to see what it is you're selling with that as your representative.
A live-action mascot is what we'll call the types of mascots you see sports teams as having -- someone dressed up inside a massive costume, sweating all their water-weight away while they pantomime to fans. You can have a live-action mascot of your own without investing in a heavy-duty costume; just go to a thrift store sometime near Halloween and pick up a lightweight costume for cheap. You can use it year-round and build a consistent "fan base" in your flea market customers, too.
Human mascots are great if you have a product or store name besides your own. For example, if you sell homemade shoes and call your business "Freddie's Fun Footwear" (and your name is not Freddie), you can find a costume of someone who might need awesome shoes. (Perhaps a cowboy, an astronaut, an athlete?) Then, give the costume-wearer that name and have them take on the persona of your product's namesake. In this example, Freddie could have a spiel about how he always had sore feet until he wore those shoes.
Animal mascots are particularly attractive to children, though adults are drawn to them as well. Something about a person dressed up as a brightly-colored animal really draws attention to your cause or stall, so if you can, find an animal that expresses your mission or product well and have a friend or family member don the costume each market day to impress your potential customers. Some classic animals: housecats, dogs, wolves, lions, tigers, mice and other rodents, birds, butterflies, bees, dolphins, and horses.
For a variation on the animal mascot, bring your pet along to the flea market, if you're allowed. Leashed animals are often allowed on flea market grounds, if they stay away from the food stalls, so you shouldn't have a problem with this. Live animals are a great attraction and draw people to your stall, especially if you have an unusual pet of some sort, and you can explain that the pet is your mascot to get some "aww" noises out of your would-be customers.
How do you find your flea market and garage sale opportunities? Have you tried our main site, GarageSaleCow.com? We've got tons of great free resources we'd love for you to use.