Hi, and welcome to the forums.
What do you mean by "what do I look for"? Are you going to be next to the phone when purchasing it (so can look for damage, check things are working)?
One thing to look out for, is to make sure that if the SIM card you already own will work in that phone. For a phone to accept any SIM card it must be unlocked (i.e. not locked to a specific network). So, your SIM must either by T-Mobile (assuming the G1 was originally purchased through T-Mobile), or the phone must be unlocked.