If you got it from a pawn shop, it could be stolen, and if they can't supply you with the password, you should take it back because it will be useless to you. Without a password you won't be able to download apps, store passwords use some system preferences or much of anything. As mentioned, not even having the original installation disk won't help if there's a firmware password.
I'd also be suspicious that nothing was wiped out. I don't know anyone who would just sell their computer without removing all their files and personal data, if not actually doing a totally clean reinstall. If there is a firmware password, I would guess that it is definitely stolen, because I doubt anyone who has bothered to go through the process of setting one up wouldn't forget to disable it if they were selling it just a year later.