I am developing a website that will deal with buy and sell mobiles business. Most of the pages of website will generate by users, for example a person A posts an ad for his used Nokia Lumia 800 and in description he mentions that mobile is in good condition etc. and at the same time another person B posts an ad for same mobile Nokia Lumia 800 and he describes his mobile almost same as Person A described, so that will generate two URLs(webpages) referring to almost same identical content.
This is what Google says about the issue (If your site contains multiple pages with largely identical content, there are a number of ways you can indicate your preferred URL to Google. (This is called "canonicalization".))
So, Google is asking me to provide preferred URL for my identical content but in my case both URLs are completely different in nature, there is identical content but both are different ads.
Note: I am not allowing users to describe condition of their mobile in long text, instead I am getting info of mobile using check boxes and drop-down menu.
Please help me that how can I deal with this situation and what is the best recommended SEO practice for this situation.