Dear Sir/Madam,
This is to inform you that on (date) I bought (item) from you for the price of (----). Unfortunately, I have discovered that this (item) is faulty/not fit for its purpose/not as described. I am, therefore forced to reject it and request you to refund the amount of money that I have paid for it.
I shall look forward to hearing from you at the earliest possible.
Yours faithfully,