Gutted for you Kev.
Be interesting to see if feedback is left from a buyer. Just to add a little more to story, the seller is the salvage yard that quoted 7.5k for a repair.
IMHO I think they always intended for it to be written off and by quoting such a high amount for a repair, knowing it would reduce the value of the car to the insurance company.
Scenario - salvage yard tells insurance company it's a complete mess and offers to buy it off them at a very low price, knowing that he'll get more for it due to the minimal damage and low mileage and excellent pre-accident condition etc. Then lists it on eBay in a short term auction (before Cat C or D is confirmed, hence the reason it could still be driven away).
Dirty tactics and very underhand, but nothing technically wrong.