Mr Morlock wrote:buying and selling is about communication , negotiation and at the very least cordiality. Telling people to piss off is really unacceptable to most.
Exactly.
There's no reason to be rude in general, even more so when you are selling items.
I've had ridiculously low offers for items I have sold in the past, and I've always responded politely "thank you for the offer, but it's a bit lower than what I was looking for, would you be willing to pay closer to $X?"
it's free, keeps people happy, and is more likely to get a sale than "piss off mate"
Of course, the way the potential; buyer carries themself has an effect on how low I'm willing to go on price.
I've given stuff away because people have been really polite and nice to deal with. (actually a few close friends I've met have been ones that bought old car parts from me years ago)