Hello! I want to sell my iphone 3g because i have 3gs- i cleared the info in settings and now have black screen saying no service and usb port pointing to itunes - is that all i do ?

Hello! I want to sell my iphone 3g because i have 3gs- i cleared the info in settings and now have black screen saying no service and usb port pointing to itunes - is that all i do ? i am pretty ignorant of computers- lol- i had read that i need to plug it back into itunes to verify it has all cleared out- i want to sell it on craigs list or ebay- i am really hurting for money right now- all your answers appreciated...