It's mostly about the library. Many long-term Playstation users have large libraries of expensive games tied to it, so people buy stolen accounts so they can play all the games on them without buying all the games themselves.

Game-hackers also buy the accounts so they can get around being banned from online games, but considering the price mentioned this sounds more like a big game library account.

I'm assuming the PSN name was something short or of value. Something like "Justin" would get a lot of bidders.