What would be the rate of TDS under section 194Q if seller is a non-filer of the Income-tax return?
As per section 206AB, if the deductee has not furnished its return of income as per section 139(1​) and the due date has expired, tax is required to be deducted at a rate higher of the following:

   •  Twice the rate prescribed in the relevant provisions of the act, or

   •  Twice the rates in force, or

   •  5%​