As it says on the site, check often.
While that kind of comment is often used as a way to boost net traffic, it's also true in this case. New product arrives all the time, and is sold all the time. This goes for eBay and Amazon, too. Amazon also allows you to maintain a wishlist that you can easily check for a series of items: at a glance, you can see what used versions of the items you want are selling for.