Mr. X being a registered dealer will have to pay GST Rs.90/- on Reverse Charge (RCM) on his purchase of Rs.500/- as the same is from an unregistered dealer, for which he can take credit (ITC) of Rs.90/- in his account. Now suppose he sells the Circuit Board for a profit of Rs.50/-, his selling price will be Rs.550/- and applicable GST will be Rs.99/- @ 18%.  For this, he will pay by debiting Rs.90/- from his credit in ITC and the remaining amount of Rs.9/- will be paid in cash (bank) which will complete the transaction.