I wouldn't use a recycled H02 sensor for the upstream pos, that signal is a feed back signal for the Air/fuel mix.
As far as using a used HO2 sensor down stream, it just tells the PCU/ECU, if the catalytic converter is doing it's job!
Don't forget to use Anti-seize compound.
I have no problem using recycled parts like EGR valves, O2 sensors, bulbs, fuse's, relays, engines, transmissions, and metal body parts.
Caution - Don't waste your time with old plastic, or rubber.