Market stalls are small businesses that run in markets made up of many stalls, usually running on a weekly basis in a car park or other public venue. The possibilities for a market stall are limited only by what will fit into a stall: vegetables, prepared foods such as bread and pies, clothing, electronics and furniture are all sold from market stalls. What you choose to sell depends on two factors: what you are interested in and good at making or acquiring at a low price, and what will sell in large enough volumes for you to make a living.
Food is a good bet for making a profit with a market stall. People have to eat, and thus are always buying food. Options include buying fruit, vegetables and other produce at wholesale prices and selling them at retail, making prepared food such as baked goods or even frozen meals, and selling dry goods such as beans and rice in bulk. Drawbacks of a food stall include worries about spoilage and food safety, a lot of competition and (in the case of prepared foods) the inability to build up a huge inventory in advance.
Electronics such as computer parts, watches, personal audio and household appliances do well in market stalls. They are small, easily transportable, and have reasonably high value. Most market stalls sell relatively low-quality electronics, and a fair number sell brand-name forgeries and other questionable goods. If you sell electronics in a market, find a way to distinguish yourself from these questionable vendors. You do this by selling high-quality merchandise, being able to prove its authenticity, and knowing enough about the product to be able to make recommendations and help potential buyers make informed choices.
Some markets sell only new items, some sell only used items, and some allow everything. Setting up a stall with used merchandise is basically creating a permanent yard sale in the eyes of the public. This is fine, but don't expect to sell things at a premium. The trick to making a living with used merchandise in a market is to be able to acquire a steady and reliable volume of it and to sell it consistently. Be wary of shady dealers and be sure that you aren't selling stolen goods. Many vendors specialise in one area of used merchandise; for example, vintage clothing, antiquarian books, or antique tools.