If I have to create a UI which has a form for buying a product and that page is access by lot of users( millions of users). then what would be design and tips I should consider while desiging the UI of the page
It would be nice of you to leave a link in the "dead" thread to the "live" thread so the topic does not become scattered (and complicated). You could even ask an admin to delete (or lock?) the old threads.
And yes ... my advice, which was given my hint is this: keep it simple and follow conventions. Look at other successful sites for conventions. Keep things simple to begin with. And check your work for simplicity and intuitiveness with other folks at every stage -- preferably new folks who haven't seen the site or its previous concepts before (at every stage).