For selling a GSM phone, the #1 question any buyer might have is whether or not it's "locked" (i.e., it won't work with somebody else's SIM card). You can test this, easily enough, by finding a friend's GSM phone (preferably from a different network, i.e., trying a Cingular SIM card in a T-Mobile phone), doing a SIM card transplant, and seeing whether it works.