Down here at the pawn shop

[glatt]

In this case, I agree, since the guy was a crook. But if my mom went in and tried to sell something, and the person working the counter lied to her about what she had and paid her less than it was worth, I'd think that was unethical. But I have no problem with taking a crook.

It's the lie about what the item is that makes it unethical. And the price depends on the lie. If you can negotiate a good price without the lie, there is nothing unethical about that.