Visit the Pokémart representative next to a house. He will tell you to follow to the Pokémart and tell you that you can buy goods from the Pokémart. He will give you a promotional item, Potion.
The price for items at the Pokémarts are consistent at every shop across Hoenn.
Blocked Route 102
You cannot access Route 102 as it is blocked by a researcher. The scientist is sketching what he believes are footprints from rare Pokémon. To access it, you must go to Route 103 to defeat Prof. Birch's child and come back to the place again. The researcher will step aside and say "I am finish sketching the footprints of a rare Pokémon, but it turns out it was my own footprints".