Did you have the bike MOT'ed while you owned it?
was the MOT currently on it before or after the accident?

With Ebay, there are no comebacks, I know this as I bought a jetski, described as needing a service, stopped running.
And since then found that engine needs a complete rebuild, had broken piston, rings and melted piston on 2 of 3 cylinders, and also damaged crank.
There is no way to get money back off ebay sellers, even in my case when the ski was completely mis-described.

As others have said, its the buyers responsibility to check everything, and get an inspection carried out at their expense if they want to be totally secure in the knowledge that the vehicle is what they are hoping for.

As you say you didnt know, and havent tried to purposely conceal any hidden damage, you have been honest about the drop/slide. He must have stripped the bike down to find these problems which I find very suspect in itself. Why dismantle a bike after just buying it?

I wouldnt be bothered if it was me, its not new, its sold as is..