I'm drawing a blank on this one, but I guess it depends on your definition of a classifieds site really.
Plenty of free online selling platforms to select from, such as Bonanzle and Ecrater to name a couple, but I wouldn't refer to them as classified sites personally.
The thing is with free listing platforms, costing you nothing to try, so they are a great way to increase your market exposure. From the business model you describe, I would suggest those two platforms are a very good options for you.
They are always more effective if you embark on an aggressive marketing campaign to try and generate some traffic flow for yourself, but they do generate a decent flow in general.