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!
Step 2: List Your Stuff!
Remember, you can remove a listing from the "Account" tab.
Once you list your item, you can copy/paste the URL into your Instructable.
You can also distribute your "Seller" page so Instructable viewers can see all your items in one place :)
Step 3: Buy Something!
Once you purchase an item, both you and the seller are notified via email and your email addresses and contact info are exchanged.
Step 4: Security, Etc
The site has been tested, approved, and verified by PayPal! :)
Also, if you have any questions/feedback/ideas, let me know! I created the Marketplace for our community, so I'm happy to change/modify/update whatever you guys want! :)