You don't need to "pretty up" junk silver unless you are trying to mislead someone into thinking it's better than it is. Once you get a reputation for selling "cleaned" coins, it'll be hard to live that down. No one will want to buy your stuff. And potential buyers will think... if you're selling cleaned coins, what other deceptive practices have you used on your coins? Best just to leave them alone.
Just a thought. Could it have been coated with lacquer for preservation purposes? And it was the lacquer coating that "toned". I suspect acetone would do a job on lacquer.
I suppose it's possible but I think it highly unlikely because I have never known lacquer to turn black. Even with great age lacquer usually only gets a yellowish cast or sometimes a light brownish cast to it. And yes acetone will sometimes remove it, but it can be very tough to get it all off. The tiny little protected areas and recesses on the coin will often retain the lacquer coating even after extended soaking. There is another possibility I just thought of. If a someone tries to clean a silver coin with ammonia the ammonia turns the coin black. But, I've never attempted to remove that black so I don't know if acetone would take it off or not, but it might. And ammonia would also explain the smell he experienced.