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 for OBDIIC&C, IMAC&C, BCMC&C, SimpleCycler or the IMABOOST are $55 each inc p&p.
Pcb's for Current Hacks or the HCH1 Fan Control are $30 each inc p&p
Paypal me at 150mpg[at]gmail.com