Whilst I am not clear from your post whether you bought the item from Amazon or Mabtech the following may help:
The review of this item available click here makes clear that:
1. The response time is 25ms and is therefore not suitable for games.
2. To get the best PC quality one should use a DVI connection.
3. To get the best TV picture the contrast and brightness have to be manually adjusted.
If where you purchased this item advertised it with a response time of 12ms and the 'real' response time is 25ms, as this review states then the item you have bought is not as described and can be rejected.
You have raised this matter and have not used the item, therefore it would be hard to argue that you have 'accepted' it.
I would write by recorded delivery to the retailer making these points, rejecting the item and asking for a refund. If you paid by credit card I would contact the card company enclosing a copy of the letter and asking for their assistance as they are jointly liable.
Of course under the distance selling regulations you could have returned the item within the first seven days anyway.