You have had the mobo for nine months which is a long time in retail terms.
I agree it should not have been replaced with an apparent used version (check the Scan terms and conditions for its replacement policy), but I have used Scan over many years and generally, like thousands of others, have been pleased with the company.
I've also been to the premises (at the rear of the Reebok Stadium in Bolton) and, at the time, it stated that any motherboards returned that were found NOT to be faulty when checked would be subjected to a test charge. This was quite hefty to ensure that only genuine complaints were received.
I would write to the MD privately and confidentially and point out you are very unhappy at the way you feel you have been treated.