In my experience, squash and pumpkins always sell well. They don't need a lot of fussing over either. People love acorn and butternut squash especially. Potatoes are also fairly easy to grow, but are cheap unless they are organic. Peppers and tomatoes will always sell, but the plants need tending, and if you're growing hot peppers you have to make sure to stress them so that they actually get hot. Onions, beets and radishes have a quick turn around. I would stay away from leafy greens unless you're really going to get into the business of producing food, as lettuces etc. don't keep long, and can also easily transmit food-borne illnesses. Hope that helps!