Yes, you may advertise many different products, but don't "spam". If you "spam" the free classifieds, all your advertisements on the spammed site will be deleted by the site owners.

The best method is to use different e-mail addresses and URL's wherever possible and create an effective advertising regimen. Since ads on the free classified sites rotate off in a matter of days, post the SAME advertisement to the 15 and 30 day sites every 5-7 days as a minimum to keep your ads current.  There is a more detailed submission schedule within the help under the "Beginning Instructions" section.