Say if i am using free classifiedes for promotion of my site, some conserns are that it might get banned if google looks at it as if it is spamming. Buy submitting different product reviews with an identical url-- will that too be counted in spaming or google will allow me this.
No you will not get banned for placing links on free classified websites. It will only benefit your website. It is spam, but it is ok if they are allowing you to post it.
Since the links point into your domain and you do not reciprocate them I wouldn't worry about them hurting you. If you use automated software to "blast your ad" to thousands of these sites at once you may trigger a filter at Google... looking like you are involved in link farming due to the speed at which you acquired these links. How fast is too fast, how many is too many, how many is too many too fast? Welcome to the world of SEO... unfortunately, no one on these forums short of a search engine engineer is going to give you a solid answer. The best case scenario is you build a little link popularity for your site and may make a few sales through the classified placement. ...If you visit each site and manually enter your ads I don't think you will ever run into any "link farming / SPAMMING" penalties.