I am newbie when it comes to selling photos. I just wanted to enquire if we can sell photos on multiple stock sites?

Also, is it possible to keep my photos on flickr and upload them on ShutterStock? Do they require me to pull down all the photos from flickr or any other free image sharing service?

Unless they are very heavily watermarked, I do not advise you keep them on Flickr. You would have to see the Terms of each individual site to see if there are any express prohibitions against it. For instance, you may be in conflict if you have Exclusive images marked at Dreamstime and posted at Flickr.

Plus be careful on Flickr that you did not mark anything under a Creative Commons License. Putting them under a CC license makes it unethical and probably even illegal under the terms of the CC license to then sell as RF. A lot of people carelessly mark a CC license withut knowing exactly what it is.

There's lots of stock stuff on my blog

http://niagarashooter.blogspot.com/

And doing a search on "microstock blog" or micrstock news will bring up several of them. On my own blog, I have lots of stuff for newbies. And if you want to explore deeper be sure to see Microstock Diaries, which is in my link list there.