I want to sell my used generator but don't know how much to sell it for, is there a website that can guide me as to how much I should charge?

Try checking Ebay. Whether you have a seller's account on the popular website or not, with a few clicks you can enter in a description of your generator and see how much ones similar to it have been selling for online. The "buy it now" prices listed are a good indicator of the median selling price.