I recently bought a fairly new HTC G1 after a long tiring search for android phones at Computer village, Ikeja (Lagos, Nigeria). Most of the phone dealers had only windows mobile phones, iPhone and Blackberry, but no android phone. Just when I was about giving up, I came across a dealer that had a fairly new HTC G1 (he called it London-used but it's obviously from USA). Anyway, I bought the phone and if not for the Motorola charger that came with it I could have easily passed as new.
Since the phone isn't new and the former owner did not do a factory reset and had the phone registered with his gmail account. I had to change the gmail account to mine to be able to access the android market (the phone requires me to enter his account password to access the market). I didn't want to do a factory reset so as not lose any application installed by the former owner. Actually, I have an aversion for factory resets because of the painful data loss they often cause.
After searching the internet for another way of changing the gmail account that will not involve any multiple system settings change. I came across a method that worked fine and is probably the only alternative method.
So outlined in a detailed manner below is this method of changing the gmail account -
Goto Settings, Applications, Manage applications, and browse to Gmail storage, select Clear Data. Also goto Gmail and select Clear Data. Finally goto Google Apps and select Clear Data.
Now go back to Settings, and select Data Synchronization. You will be prompted to setup a new account. Enter your gmail account and password. So now, you will be able to synch your gmail on your phone and also access the android market. You will also receive a mail from the android team welcoming you.