Perhaps, but my tendency is to appreciate that if I buy second hand I buy at my risk. If he didn't deliver the item in the photo or it was clearly knackered and the photo didn't show this then you are right, he is a twat.
If on the other hand, you got delivered what is in the photo then that is your look out. He offered to take ten item back, so you would not be out the item cost of a useless piece of kit (although others have commented on a perfectly useful work around) but to ask him to pay for the postage is actually a bit rich.
The dealer might not have been the most diplomatic in the world, but actually I think it is you who is being the twat in this instance.
Well thanks for your ten pence worth, you would'nt happen to be a freind of the dealer by any chance!! Have you actualy read all of the original post?
If you can come to the conclusion that I'm being a twat then maybe you need to make an apointment with your local shrink!!