Over the years I've bought cows private and stockyards, registered and commercial and it's about the same either way, if they are selling them it's a good chance they are culls. There are some good herds around but they don't readily let go of them unless it's an old cow ready to cull or a younger bottom end cow. If you do come across good young cows it's out of our price range to chance.
It's a longer program but we've had better success with buying heifers.
And then there's my partner ( wife ) who has been in on some of the cow buying and said no when I mentioned the angle of buying cows earlier today