Adventurers cannot sell mundane armor, weapons, or adventuring gear unless the Dungeon Master allows. Even then, they receive only one-fifth of an item’s market price. Art objects or fine goods that have a specific value, such as a gold dagger worth 100 gp, bring their full stated price. The sale price of a magic item depends on its rarity; see “Magic Item Values".
Published in Rules Compendium, page(s) 265.