As the O2 sensor is such a vital part in a manual G1 I would not buy one secondhand. You never know what you are getting. A new one will give years of trouble free service.
Pcb's/Kits/Built Units for ObdIIc&c, Imac&c, SimpleCycler, ImaBoost, Current Hacks & HCH1 Fan available.
Enqs to 150mpg[at]gmail.com