The Poké Mart is a store existing in all Pokémon games that sells supplies for Pokémon training. What the store sells changes with where you are, but most carry Poké Balls and status healers. Most stuff is produced by Silph Co. or the Devon Corporation. A deliveryman appears at the Poké Mart when a Mystery Gift is received by the game. They also pay for items like Apricorns and berries. Most cities have Poké Marts while the home towns do not. Poké Marts also aren't as common in the anime series. You'll find you can pick Apricorns and sell them back to the Poké Mart for cash.
Poké Mart Items
This list is generally based on Generation I, some things, like TM's change with the Generation. Generation II+ indicates Generation II on, and Generation III+ is Generation III on.