If it's some sexual part of you, it goes on Craig's List, I think
If it's something else: I'm aware of at least one guy who created a website which summarizes eBay auction prices for particular items of equipment over a much longer period than you can search.
There are various similar or different price guides to all sorts of things, but I don't know if there's one for
everything, so you probably do need to be more specific about what you're trying to sell.