Your talking about O2 sensors. You don't replace those with used sensors. And get good quality sensors. You get what you pay for and when your fuel mileage goes down the crapper you will be glad you got the good stuff.
96 XJ, 3 in. Rustys full pack lift, 4.0, AW4, D35, C8.25
86 XJ, 3 in. RC, bb rear pack, 3.4 L engine