Never pay for an unlock or JB. The companies that sell those products just package up the ones that are made freely available to everyone. Its it bit like someone blowing up a balloon and selling you the air...
With that said, which iPhone are you buying... a 3G?
If so, you should update your iTunes to the latest version, then restore your iPhone to 3.0 and set it up as a new phone.
At this point, you should download and run redsn0w 0.8. The JB process is pretty straight forward... but if you want a tutorial for how to use this tool, just view this one. I realize the tutorial is for redsn0w 0.72, but that doesn't make a difference to the steps taken.
Once your iPhone is jailbroken, you need to launch Cydia and let it perform any updates it requests. At this point, you need to add add the Dev-Team repo so you can install ultrasn0w.
Can someone please jump in with that info? I need to leave for work in like 1 min or I'm gonna be late. :-)
But anyway, what I typed up gets you 99% of the way there.