In my spare time, I created a site that allows us to do just that! You can see the featured post in the forum here, but I thought a nice how-to article might help, as well!
The marketplace is a third party site, but I think it is great because you can put the link to the items you're selling in your Instructables, browse other user's items, etc :)
Step 1: Setting up your account
It is a third party site, so you'll need to create an account. If you use the same username as you do on Instructables, it makes it easier for everyone!