When you see a keg for sale missing a part, particularly posts, step back and consider that the previous owner may not have solved that problem either. Not usually a good deal if you have to replace the posts. If it doesn't have a lid, it'll cost you for the lid as another example. In both cases, you can't pressure test it to know whether the keg is broken completely.
I am not sure if Cornelius made this kind, but some kegs have thick plastic washers under the post. I've had a couple of these, not sure what I have left. For some reason I am thinking they are one sided with the fat washer but again not sure. I have wondered time to time how replacing the posts with universals would work out.
Universal posts aren't universal. Sometimes the threads vary. I am not sure if you could thread it down that far if this was the case.