Nitin Bhandari:
1) Does Liferay supports E-commerce application development.
You can develop any kind of web-based system on Liferay, so "yes".
2) Does KonaKart gives flexibility to develop e-commerce website.
Never used it. I know it provides shopping cart support, but don't know if it has support for payment processing, etc.
3) What are the web (e-commerce) applications that are already running on Liferay?
No idea, but that doesn't mean they do not exist.
4) Any custom plug-ins that are available in liferay market place?
You'd have to check, there's new plugins being created all the time.
5) Any out of box features?
Not that you'll find support for specific ecommerce handling.
The biggest issue is the payment processing stuff. You have to have a vendor that you are going to work with for credit card handling, etc. Once you have that you can start building layers around it. Konakart might help with your shopping cart support, but you'll still have to do your basic product listing, search, detail view, etc.
Well, I guess if you built custom entities using SB you could add Liferay comments, ratings, etc. so maybe there is some OOTB support that you can leverage.
