There should be an NPC at the Marketplace in London that allows you to sell items easily and quick, hence the name "Quick Sell." All items would have a set price that doesn't change, and when you Quick Sell the items you receive that amount of gold. The purpose of this feature would make it easier to sell common items that you don't need, such as cobblestone or grass, that people don't need. This wouldn't destroy the purpose of the Marketplace because items would be worth more on the Marketplace. The point of this feature would be to have a quick and easy way to get rid of items you don't need.