Welcome to the Q&A Forum

It would help to use the canonicals as you describe and it is proper the way you mention, but because the products are up for only 4 weeks, the new pages probably don't even have time to rank.  It would be kind of a moo point http://www.youtube.com/watch?v=fLwYpSCrlHU You need to probably focus on category pages that might point to the items as those will be more static and can build rank.  They can point to the "sold" archive item page and then when a new item comes up will already have rank on the related category to make the sale.   We have used this on job boards as it works the same.   Any given job will be posted for 1 month at the most, but the category page of $type job in $city will rank for that key phrase and then point people towards the ever changing job listings.   Not sure what you are selling, but think about how you might be able to setup an evergreen type of topic page that people would search for related to your products and focus on that.