I finally got a locksmith to come over and get the lock open. It turns out the keys the previous owner gave me, the key that was supposed to open that particular lock didn't match. The locksmith said it was difinitely the wrong key. I took a closer look at the door and it was a little cracked on the side plus the locksmith said the lock wasn't of the same series as the other locks in the apartment. The rest of the locks in the apartment was silver color while the odd one out without the key was gold in color.
I think what most likely happened was the tenant of the previous owner, at one time or another, probably got himself locked out of the room and broke the lock, hence the damaged door side. After that, the guy replaced it with an identical lock... only he forgot to return the new key to the previous owner.
That so pissed me of. I spend a few hours trying to get the lock open. I even sprayed it full of WD-40 to lubricate it. Damn it.
I think I'm going to make a bonfire with all the previous tenant's old mail.