Okay, I was considering buying this iPhone 3GS however it is brand new and the seller does not wish to open the box. He did give me the serial number however, so I was wondering what baseband/firmware I should expect out of this phone:

879504PQ3NQ

If I'm reading that correctly, this is week 50 2009? I think this would be a tethered jailbreak but I don't really mind, I just need to make sure it can be unlocked.

I got mine same week and had 3.1.2 and baseband of 05.11.07.
You should be able to jail break and unlock.

As the above poster mentioned, you should likely have the 3.1.2 Firmware on the device and the 05.11.07 baseband, so it should be jailbreakable and unlockable. Most likely it has the upgraded iBoot so your jailbreak would be tethered.

Sweet success