i dont think its actually worth buying the new iphone 3G
in the black market bcos its gonna be waaaay more expensive
& as far as ur question concerns, u can use it
with any network cos u can use a sim card with it, i guess
take a look at the following site:
http://www.gsmarena.com/iphone_3g_for_$199_or_even_free__but_whats_the_catch-news-521.php