You can sell items at any pokemart.
Don't sell your Nuggets or Star dust at Pokemarts. If you hold onto them until you get to the 7gym town. You can Sell it to an old man in the Pokecentre for Double what the pokemart buys it for. Unless you're tight for cash I'd hold out for him.
You can't sell it at the poke mart
Try anvil town