Due to popular demand, the marketplace forum has been replaced with a new marketplace that is not a forum but a purpose-built place for buying and selling.
The old marketplace forum is still there, and people can still respond to the old discussions there, but new ads should be placed on the new marketplace.
There's a "Marketplace" link in the horizontal menu at the top of the page, and a box in the right-most column showing the last 10 ads.
The new marketplace has a few bugs, but we hope to iron them out over the coming weeks. Let us know what you think.