For an example assume my gig is for write articles. So when a buyer contacted and gave all the details, is it OK to start writing articles or should I wait till buyer actually bought the gig?
Clearly you can do as you see fit.
I suggest doing nothing until the Buyer presses the “Order Now” button and submits the gig.
Do like @kjblynx says and be polite, but be firm as well.
You should wait until the buyer has bought the gig and submitted the order requirements ‘again’ in the order page.