The rude thing is to not offer any reward in return for you, if you agree to their request.

It's just saying they want to take advantage of you if you fall for it. Making such a request means that they are happy to take advantage of you as long as you let them. Is that unethical?

Think of it this way: You own a truly valuable stamp but you don't know its value. Then somebody who knows its value offers to exchange it for their stamp of much lesser value, without telling you what they know about its value.

It may not be unethical, businesses are based on such behavior. Buy low and sell high to make a profit. But when you see such behavior by your friends or neighbors or colleagues, be aware. They are the kind of people who are happy to take advantage of you.