I don’t see what’s so unethical about it. The RE Agent is an independing contractor. He can do whatever he wants.
In my book, it’s OK to discriminate based on the money and time involved but not based on the person.
Buyers need to educate themselves some more. They have the money so, in essence, they control the transaction. If they don’t like something, they have the option to walk. Keep the emotions out of it.