I have a sum of money which I would like to invest. Is it permissible for me to trade with this money so that whenever a commodity is demanded from me I buy it and then sell it either in cash or installments after adding my profit?
May Allaah Reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

This form of investment is lawful as long as the demanded commodity which you buy becomes fully your own before you sell it. Allaah The Almighty Says (what means): {And Allaah Has permitted trade.} [Quran 2:275]

Allaah Knows best.