The Black Market Auction House features a wide variety of items. One week it might have something like an Enchanted Broom, the next week Ashes of Al'ar. It's completely random, and varies per realm. Certainly there have been items up on a few realms that people hit the bid cap on.
Just keep watching, it's bound to have something you want sooner or later. Only trick is, it'll probably be desirable to other people too. Good luck!
Why does it have items that can be obtained for a much lower cost and effort?
Probably to make it seem like the real auction house.