Not that I don't think ML needs a market place, I don't think it fits with the focus of the community.
Posting you have things to sell here is perfectly fine, but adding a market place would be odd. It's super simple to tell who the real people are on ML. They may be in the market looking for something, or they may know some one who is looking and can recommend them to come buy from you.
It could also be me as I have been trying to clean out my garage for months, and yet I hate to take things like that to goodwill. I got a little bit of my own office space for my new little startup, but there the stuff in my garage would fill up a whole 1 (of 3) offices.