Is there any problem if i buy some .mobi domains which are already registered as .com extension for someone else? for example a very popular site xyz.com and its xyz.mobi is available(just for consideration) , should i go for such these domains?
So long as the person owning the .com doesn't have a trademark on the name you should be alright (e.g. don't try registering microsoft.mobi). I have some .net and .co.uk domains where someone else owns the .com and I don't see why .mobi would be any different. Of course, if you're trying to register xyz.mobi with the aim of selling it to the owner of the relevant .com and/or ripping off their site, then you're likely to land yourself in trouble.