Oxygen sensors

The problem with that is it's not normal for all 4 O2 sensors to fail at once, and if all four are a problem, then it's very possible for the converters to be bad because they are not being tested. Do you know the actual codes the vehicle has or is this a seller telling you that it just needs the sensors replaced? Personally, I would pass on it unless you could have it proven that "it's just the sensors".