It doesn't have to be MtGox, but anyone who wants to see a quick collapse in the price. And given the anonymous nature of the internet, it is super easy to do.
I don't think it's the anonymous nature of the internet that makes this easy. After all, the internet is far less anonymous than many think.
It's likely the unregulated nature of Bitcoin that makes this easy to do. After all, who is going to chase down market manipulators with search warrants and wiretaps?
