In Generation V, around the Unova region there were various item maniacs you could see useless items to. There was ore collector in Icirrus City, this is where you can take those Big Nuggets you get to. There's also a Gourmet manic as well, she's on Route 5 in a trailer. You can give her food items in exchange for money. Another item maniac is a rich man in Undella Town. He will take those items you find while using Dive underwater, and flutes you get from a random NPC a route north of him. There's also a Rare Bone collector and a Mulch collector. The Mulch Collector is only in Black2 White2, however.
Did you hand your useless items over to these item maniacs? I do, and it's really easy to get 100,000 pokedollars from it if you stack up enough of them. Especially the ore maniac. He gives you double if you hand him Stardust, Pearls, and Big Pearls. It's really quick money.