These are the items you can buy:
Pokeball - $200
Net Ball - $400
Potion - $200
Antidote - $200
Paralyze Heal - $300
Awakening - $300
Burn Heal - $400
After Getting 2nd Gym Badge
Dusk Ball - $1000 - Greater Chance of Catching a Pokemon at Night or in a Dark Area
Moon Stone - $1200 - Evolves certain kinds of Pokemon