Respond by email and say you might be interested and ask for a phone number you can call. If he wants your phone number for contact, but won't give you his, then fishy. If you get a phone number but there's a juke box playing in the background when he picks up and you hear the distinctive sound of a pool cue breaking over somebody's head, also fishy.

Other than that, cash-only on CL is pretty standard, at least for me. Take a check from somebody I've never met? Ain't gonna happen. Been there, bounced that.