A skeleton with an ebony helm? Never heard of that before.
Perhaps what you saw wasn't a skeleton, http://images.uesp.net/5/51/MW-quest-Hunger_in_the_Sarano_Tomb.jpg. Its a deadra...which is basically a demon in mundane terms.
The helmet you retrieved is probably the Serano ebony helm...You're asked to kill that specific hunger for a fighter's guild mission. Consider yourself ahead of the curb.
EDIT: As to where to sell it. http://www.uesp.net/maps/mwmap/mwmap.shtml?centeron=Mudcrab+Merchant merchant who offers you original price for your goods, and he has 10,000 gold. Sadly..you will lose 5,000 if you do not own another item of that value you can buy back to lower the selling price from 15,000 to 10,000.
If that last sentence makes no sense let me explain to you how to get the most out of bartering.
Spoiler
If the mudcrab merchant has 10,000 and you need to sell something that is worth 40,000 you must stock him up on cheaper items either 5,000 or 10,000.
Once he has a ton of these you can then proceed to sell your 40,000 item.
Once you sell it you'll notice he doesn't have enough money for the transaction.
Proceed to buy back all the cheaper items you sold to him until the transaction becomes affordable for him.
Sell the rest of the items when he restocks on gold which is every 24 hours.