Trading activity can be of various types. Say for example a manufacturing unit making some steel products is also trading in steel. Trding is possible also on the inputs on which cenvat credit has been taken, i.e. by removal of goods as such.
Trading can also be related to manufacturing. Say an unit making computers wants to trade in printers not manufactured by him. He can keep printers in the same premises, and if the customer wants to buy printers also, may seel printer to them.
A good example is service station of automobiles. They sell spares and also provides services. When a customer receives bill, it clearly shows the value of sold items, along with value of services on which service tax is paid. There is no provision that the manufacturer cannot sell, both manufactured and trading item on the same invoice.
If the manufacturing unit is trading in something, which is not his input or final products, there is no problem. He can have a separate series of invoices for trading item and do the trading.