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When you first come here, you can't do much here, the house with the sign next to it is the mart but sells useless items and a few potions and PokéBalls. To get to the gym go north to the Lake of Rage but first, fly to somewhere to get some Great Balls, you have a battle with a very very rare Pokémon.

[edit] Items

Considering the town is overrun by Rockets, you won't be getting anything free... the most unique item you can get would be is a Rage Candy bar, which is as strong as a potion, and costs as much as it (300). However, any items that you can buy that are really useful can only be obtained after you drive out the rockets, and that still will be a while...