Should I be closing shops?

Does anyone know what nexon's capable of?
Like, can they see how much of a certain item someone has in their inventory or stuff like that?
Or trace down who used the hacked?

They can see everything that's in the database, so of course they can tell how many of something people have. (If they make their queries correctly... they have been known to goof in the past).
As for who used the hack, that depends on what sort of logs they keep (of incoming packets, database transactions, etc) and how long.