If people don't want to be 'insulted' by low offer then they shouldn't present it in a format that allows for offers that are 'too low'. It would take less time to reject the bid they allowed to happen than to write a response.

I have personally purchased at least one thing for 10% of the asking price (around $500) because I was the only one who wanted it and they wanted to get rid of it. That was what it was worth to me, I wasn't trying to haggle or anything, just made an offhand offer when walking by.