It's a courtesy. A seller has the right to set their price and to not be bargained with in the open forum. Better prices and offers can be discussed via PM but ultimately if someone wants a to get a price for something then it's up to them. If they don't sell because the price is too high then they can lower the price or take it off the for sale forum. By the same token, you can PM another member who is interested in the item that it is for sale elsewhere. I think it shows respect to a fellow member to not have their sale thread end up full of best offers or posts discussing the prices. This can cause a thread to deteriorate quickly.
All gear that I have bought here as well as on other forums have saved me money vs. buying new. I have bought something before for less than new and then seen the same thing for sale for less money a few days later but you never truly know the condition of the product even with pictures and it's never enough of a difference to cause me concern. I already saved money so it does not bother me.
Don't let life get in the way of living.