Possible Auction House bug
I’m not sure what you mean. You won’t actually sell at lower than the highest buy offer — if you try to confirm the lower price, you’ll still end up selling at the highest buy offer price.
If you are referring to something else, it would be helpful if you could make your example more specific. If you can’t replicate the problem exactly, you could make a mock-up screenshot, and explain what you saw that was different from the usual.
John Smith: “you should kill monsters, because killing monsters is awesome.”