2001 Toyota Corolla P0420 OBD Code: Is It a Serious Issue or Just a Faulty Oxygen Sensor?

Hello, I'm looking to sell my 2001 Toyota Corolla, but the check engine light has come back on with a persistent P0420 OBD code. To be transparent with potential buyers, I'd like to understand whether this issue is serious or minor so I can price the vehicle accordingly. Here's what I've observed: about 10 months ago, the check engine light activated and I used an OBD-II scanner to identify P0420. After resetting the code, there were no further issues during spring and summer. The light reappeared in October but went off after approximately six weeks. It has now returned again with the same P0420 code. My fuel mileage has remained stable—no noticeable drop over multiple tank fills. My question is: does a recurring P042 or P0420 code typically indicate a failing catalytic converter, which would be expensive to repair, or could it simply point to a faulty oxygen sensor? Edmunds.com estimates the vehicle's value at around $3,500. I'm considering listing it for $3,200 and being upfront about the P0420 code. Does this price seem reasonable given the issue? Any insights would be greatly appreciated. Thanks in advance!