Whether Security Transaction Tax collected is taxable under GST?

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No service tax on stamp duty and Security Transaction Tax (STT)  – Stamp duty and Security Transaction Tax (STT) is liability of buyer and stock broker pays it acting as pure agent of the customer. {FAQ No. 77 issued by CBIC on banking sector on 27-12-2018}

Transaction charges collected from clients and paid to stock exchange  – In Shreyas Stocks (P.) Ltd. v. Commissioner of CGST [2018-Chennai], assessee, a stock-broker, collected transaction charges from clients and remitted to stock exchange. It was held that said charges would not form part of gross value of taxable service.

So, above points help you to understand “Pure Agency” concept in GST. This concept is adapted from erstwhile Service Tax Law.