High quality is essential to keeping the market attractive to weekly customers. Commodities are best when picked the day of the sale. Customers like to see carrots with the tops on and melons that are fully ripe. You need to have a wide variety of products. If you have a display that offers only lettuce or carrots, you'll draw fewer customers than if you sell both carrots and lettuce.
Colors are an attractive draw.
Stands with three or four items seem about right. Organically grown crops are popular at the market. It is possible to sell too many items if you don't have very much of each. People like to see that you offer a lot of each vegetable. The higher the heap, the better it sells.