If someone is too uninformed (or possibly "stupid") to research where to get something for cheap, it is not unethical to sell them a higher priced product of the same caliber.
I do think it is unethical to try to snipe somebody's customers out of spite. Whether it is directing them to another retailer, or selling to them directly, it isn't a good practice.
If somebody was really getting "ripped off" by an unethical, misleading trader with incorrect product description or bad trading habits, I would be all for tipping off a potential buyer. Tipping them off to cheaper prices during an auction isn't acceptable in my opinion.