So I've searched this entire forum trying to find the answer, but still no luck. I'm selling my iPhone 3G on ebay right now but don't know what I need to do to ensure that the buyer can use the phone on his/her account. I am staying with AT&T, so I am planning on switching out of the iPhone data plan when I get another phone. So I keep my SIM card, sell the phone on ebay, the buyer puts their SIM card in the iPhone and calls AT&T to switch to the iPhone data plan? Is it that simple?