From our FAQ. Even though it says buying from eBay, it pertains to buying any used BB from any source.
Buying a BlackBerry off eBay - BlackBerryFAQ
As for unlocked, if it is unlocked, then you can use it on any network, as long as the technology is compatible. The 9800 is a GSM device so an unlocked 9800 should work on any GSM network. Depending on the carrier and the frequency bands used, you may or may not get 3G speeds, as you might only get EDGE.
The Forum charges around $9 US to unlock, I think.