The reason is why people often react to for sale ads is because they are not as innocuous as many people would have you believe. "If you don't like the price, then don't buy it" some people say, in an attempt to educate those who threaten to derail a for sale thread. But we are not just spectators here, watching someone's for sale ad attempt to appeal to somebody else. These for sale ads are for us...they are intended for us. And when someone attempts to entice you into a sale with what you consider to be an unreasonable price, then it can certainly be insulting to some people.

If someone shows up to your door and offers to sell you a pencil for $5, you would have a hard time biting your tongue. You would likely say, "dude, do you think I'm an idiot ? I get my pencils for 12 cents." You probably wouldn't just say "no thanks" and send him off to your neighbour. If you think a price is unreasonable, it is natural to conclude that the seller is hoping you are gullible enough to not find it unreasonable.

Sellers are just going to have to expect a certain amount of backlash. The fact that some sellers get personally offended by some of the responses is proof that asking someone to pay x dollars for something is not a completely objective, harmless endeavor. It is an activity full of judgement, expectation and emotion. Be prepared to deal with it.