Just answering the question as you asked it.
“My example of a ‘bad deal’ was somebody selling an apple for 50c, and somebody else thinking it’s a bad deal because they’re not willing to pay more than 25c for it.”
In the example above; the person on the receiving end of the offer has made the judgement that it is bad and says nothing about the one offering it thinking it is bad.
Your question, however, was worded differently. You asked; “It is unethical to offer a bad deal? Why?”
Earlier I wrote: “When I read just the above, I get the impression the person extending the deal has made the moral judgment the deal is a bad one. They have made the offer of the deal and the judgment it is a bad one. That does sound unethical to me. The offer hasn’t even reached my senses in its entirety for my judgment to come into play regarding its equability or its subjective moral nature. The word “deal” itself has a connotation of being good, as in the expression that was a real deal. If the person extending the offer has reached the moral judgment it is bad, it rather negates it as being a deal.”
And you respond with this?
“Anyway, I don’t think you’ve noticed that we seem to be using the term ‘bad deal’ differently – or rather, I think you have noticed, and you just haven’t bothered with trying to correct the situation.”
So essentially you are no longer asking why offering a bad deal is unethical. You recognize it. Thanks.