Can you explain by example when a payment would qualify as made as ‘principal’ (i.e. tax applicable) or pure agent (tax not applicable)?

X contracts with Y, a real estate agent to sell his house and thereupon Y gives an advertisement in television. Y billed X including charges for television advertisement and paid service tax on the total consideration billed. In such a case, consideration for the service provided is what X pays to Y. Y does not act as an agent on behalf of X when obtaining the television advertisement even if the cost of television advertisement is mentioned separately in the invoice issued by X. Advertising service is an input service for the estate agent in order to enable or facilitate him to perform his services as an estate agent. Therefore, this is taxable supply under GST law.