In Iccirus City Pokemon Center, there's a rock collector who will buy any rocks for double the price.
On Route 5, there's a caravan with a woman who will buy food items for double the price.
On Route 18, there's a house with a guy who will buy Rare Bones for double.
In Undella Town, in a big house, there's a guy who will buy the Relic items hidden in the underwater place for double. This brings in really big bucks. (total over a million)
And of course, Poke-Mart!!!
Hope this helps!