The flea market is my favorite! It's a lot of fun. A couple of tips that I've learned:
1. Keep your eye out for rare collectibles! Townies are always selling rare stuff off for cheap. Even if you weren't collecting, you can turn around and make a profit off of them.
2. If you're high skilled in baking/cooking, baked goods sell very quickly. The best advertisement is the yummy smell! Townies are drawn to it like a moth to a flame.