Value of supply of agent

When agent is selling goods by putting the GSTN of principal than he does falls under defination of Agent , he will cover under Intermediatary
According to sec 2(13) of IGST Act, intermediary is a broker or an agent who arranges or the supply of goods or services or securities, between two or more persons. But an intermediary shouldn’t supply such goods or services or securities on his own account.In this case the Agent will be Raise bill for commission to Principal (Not concern what is ultimate selling price)

Now your 2nd Query If Agent supply such goods in his own GSTN .

As per the Valuation Rule 29 of CGST Act :

The value of supply of goods between the principal and his shall be:
The open market value of the goods, or There is an option where value can be taken as 90% of the ultimate selling price by the agent, the customer and agent should not be related.
Lets understand this with an example.
Suppose Mr. A is supplying goods to his agent who sells those goods for Rs. 10,000. Another seller is also supplying same goods to agent and charge agent Rs. 8000.
Mr. A can either consider value of supply Rs. 8,000 or exercise his option and consider value to be 90% of 10,000 = 9000/-.

Note ;: kindly refer 57/31/2018 for a detail explanation on agent

**Its circular 57/31/2018 GST ,dtd 4/9/2019