We did talk about this once before. It was pretty much decided that there are other outlets for this (ie. in studio boards, craigslist, kijiji, ebay, etc.) and that it would be more difficult to police (ie. spam) than it would be beneficial.
Most people looking to buy/sell would be students so a local in-studio place would be a better place. For others, given core is international, it is not that useful most likely.
R