Pokemon Marts, or as they are usually referred to as, Poke' Marts, are buildings scattered across the map that can be identified by their small blue roofs. Every Poke' Mart has its own shopkeeper that can be spoken to. Poke' Marts are also usually located near Pokemon Centers.
Poke' Marts only have one purpose: to sell you items that you may use on your journey. When spoken to, the shopkeeper will will bring up a menu on your screen displaying a variety of item options. The only items that can currently be bought in a Poke' Mart are different types of pokeballs. The price for each kind of pokeball may vary, and most pokeballs can also be found in the VIP shop for a discount.