It sucks cause I think the legal argument will end up being that the account information is your property, but the Content IDs you buy are owned by SE and licensed for your usage only which is the only thing your payment to them guarantees - usage. The account merely grants you access to the services. So losing your account information means that you lost your own property, detaching SE from any responsibility in getting it back, leaving you up to your own means to determine who hacked your account because they have privacy policies that wont allow you any information for an account you 'don't have ownership of.' In the end it probably isn't even worth it to try to get through to the FBI computer crime site or whatever it was they directed you to. You'll probably end up spending more money/time trying to prove it than it is even worth. It really sucks but it is probably the truth.
I'm really not sure if a court of law would follow the 'black market' of currency sales. While there is a price tagged to the currency/accounts(even achievements) based on demand from those websites, I don't know if those types of things would legally be able to be recognized, as it is not really a retail value type of thing. Its one of those things where the value of the account is only as much as someone else is willing to pay for it. Legally it doesn't have a price tag. In essence it is virtual data that doesn't even belong to the person selling it if you are going off of Content ID's. If you sell your account, the account doesn't have value, the Content ID's within do, and that is property of SE if you discontinue the services. If you legally shouldn't even be able to sell SE's property, I'm not sure legally they would be willing to place a price tag based off a account/gilselling website.
I'm not really well versed in law either, but from the few introductions to law I've had theres a lot of fine print that would probably halt any attempts to try to get the account back

If you could prove who the hacker was and prove they got into your computer, prove they stole your account information and all of that and have some sort of request to SE after all of that to get your character back I'm sure they probably could, but you're going to have to be the motivating force to push the investigation, cause unless they're told by a legal source with confirmed proof and documentation that the current owner of the account obtained it illegally, they probably aren't going to budge to try to change the information.
I'm really hoping theres some legal loophole that someone will be able to find that succeeds in the account argument.