Does GST is calculated on previous added GST for the same product GST will be on transaction value

mack (Thinking-to-start-business)   (154 Points)

10 December 2022  

suppose I buy one product from person "A" on a purchase price 118/- in this 18 rupee is GST now I add my margin rupee 32/- on top of it and sell in the same state, so the selling price by me will be 150/- after adding 118 + 32.

now I have to pay 27/- which is 18% on this 150/- selling price as GST to the government.

and this is how it is said that "GST will be on transaction value". 

But here person "A" gave 18/- as GST to the government and now I am giving 27/- as 18% of my selling price which also includes the previous GST part which is 18/- rupee so the government is getting GST not only on the product price but rather previously added GST part also which is added with the product price? so technically it is not correct but people say this is the way how GST are collected by the government.

Please explain to me if my understanding is wrong ?