[lkillen]

If you WiFi to a local router (even in a StarBucks), your phone will be issued a non-routable IP I.e. 192.168.XXX.XXX but the IP address that the other end will see is that of the router, normally. Even it could be using a non-routable in certain cases. At some point upstream, a static IP resides and that is the one that will be sent. It is resolved back to your phone via NAT.
As far as 3G/4G, I guess you are SOL. I was not aware of the that limit from Craig's list. Something does not seem right. How do they know you are NOT living in Kansas? You are simply an IP address to them. That is a big Hmmmmm...