Does anyone here have experience in implementing in game payments , does such thing is possible ? Where should I look in to?
You should be able to implement Paypal payment using their SOAP API ( http://tinyurl.com/6k447de ) and any C++ SOAP library such as Apache Axis ( http://ws.apache.org/axis in C++/Java ) or gSOAP ( http://www.cs.fsu.edu/~engelen/soap.html in C/C++ ). Google Checkout doesn't seem to support SOAP at the moment through.
However, I think that the easiest and comfortable way to implement in game payments is by adding an html layer into you game ( Qt Webkit is such an example: http://doc.qt.nokia.com/4.7/qtwebkit.html, and Awesomium: http://awesomium.com ) , and to develop an online item store on your website. Thus players can browse and buy items from inside and from outside your game using their account.
Anyway you will have to connect your server to save the items bought by players, so they can delete/crash and reinstall their game without loosing what they paid for.
Note: I never tried it myself, those are only hints to help you in your quest as nobody answered yet.
Edit: I added Awesomium ( http://awesomium.com ) as example for html layer api. "Awesomium, is the first full featured, windowless web-browser framework". It seems very interesting, powerful and clean. Furthermore it's Free for Indies companies (under $100k income last year).