Canonicals

[WhoWuddaThunk]

We have a client that has his products listed on 20+ different websites, including 4 of his own.  Also, he only has 1 of everything, so once he sells it then the product is gone.

To battle this duplication issue, plus having a short internet lifespan of less than 4 weeks, I was wondering if it would be a good idea to canonical the products back to the category page.  Kind of like using canonical tags on your "used blue widget" and "used red widget" pages back to the "used widgets" page.

Would this help with the duplicate content issues?

Is this a proper use of a canonical?

[CleverPhD]

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.