This guide is ancient, but I can't work out how to delete it, but please ignore it.
If you're buying a K800i, please make sure that it is either unlocked (very few are) or is on the network you want to use.
They currently cannot be unlocked, your local phone shop or market trader will not be able to unlock it, the only people who can currently unlock them are your service provider and they will usually only do it if you are on contract and have gone through the term, i.e. 12-18 months.
If the seller says they can get it unlocked or you can get it done locally for £5-£10 then they are mis-informed.