but it was likely not a mistake right since it was such an obscure value.
They basically seem to have some backend script that auto adjusts tiv values based on a couple factors (if amazon sells it or not, how many 3rd parties are selling it and for how much, how many locked in trades there are, etc). It used to be somewhat easy to manipulate an odd item's tiv in to the stratosphere (like, say, any out of print Wii game with horse pictures on the front), but they've worked out a lot of the kinks in the last year. Clearly not all of them.
Long story short, this tiv was probably a result of the auto adjuster gone haywire - logically just consider if anyone in their right mind would pay $556 for that item used. Now consider that Amazon also needs to make a profit on it. Whether it was a mistake or not can be answered by that.
All that said, they might honor it, they might not. They've honored some crazy shit before.