For example, orbis exchange quest. First, bring pepe beaks. Second, set your item inventory as filled with garbage, pepe beaks and items you want(such as some mithril ores). Third, exchange pepe beaks for items you want. You'll be able to get another items if your ETC inventory isn't filled when finish an exchanging pepe beaks. Reproduction movie on youtube (all glitch abused items were sold for NPC)