If people are idiots I just /ignore them. It saves a lot of time.
I might go so far as to point out that the market value of the item I am selling is X and I'm actually offering a good deal. I might go so far as to link them somewhere that shows the item is a lot better than they claim it is.
there is no way in frozen hell I am ever letting anyone know how much gold I make, that just justifies their belief that I'm charging outrageous prices and they'll screen out the parts I want them to understand and focus on the price.
that conversation with me would go:
him: how much for X?
him LOL it's only 476ilvl!! wtf!!!eleven
me: it's 2nd BiS [link] and the average price is actually 18k [link]
me: I'm charging 14k, that's 4k cheaper than it should be.
If I get called a dumb derp or worse after that they get /ignored.