List of selling prices in Super Paper Mario
From the Super Mario Wiki
In Super Paper Mario, Mario, Peach, Bowser and Luigi can sell items from their inventory to shopkeepers in the four normal item shops around the world. The player cannot sell his items to Itty Bits' shopkeepers nor can they sell them at the Twinkle Mart in Outer Limits. Most items can be sold to the shopkeepers for a set price at every shop, however, some items will sell at a higher price depending on the location of the shop and the item itself. Mario can make a profit if he purchases an item at a low price from one store and sells it to another that will buy it from him at a higher price. This page lists every item and how much it can be sold for at each shop.