Gst on brass & copper scrap

I am a small utensils trader based out of rajasthan, customers who come to buy new utensils they exchange the old utensils (we can call it scrap) with the new one, also some customers come to sell copper and brass spares which are of no use.

Now how do I account this scrap and bring the stock of this in my books, so that when I sell this scrap to the factory I can invoice him accordingly.. I am very confused of the whole process.. if somebody can answer it will be really helpful.