That's a really nice nightmare, obviously. What you should do though is take a look at it, and a screenshot of it, in the classic client, so we know whether it is a shortmane, longmane ,or pure black. My guess is if it's a pure black, you'll get more for it.
If you want to sell it, I'd put it up for auction in this forum, with a 200 mil reserve, I think it's worth that, and see how it goes. But definitely take a screenshot of it in the classic client so that people can tell what kind of nightmare it is.
How much you can sell it for will also depend on what shard it is on, and whether or not you can deliver it to other shards.