Buy & Sell is trading and trading is bargain. If a seller cannot take the lower offer, then dun sell and wait for another buyer lor. The buyer is not using a gun pointing at your head, telling you to sell him your D3 at $10. So why get so workout for what?
If u want to fix a price and not allow to bargain, then go open a shop and tag your stuff with a price. Perhaps u may be the next Mustafa.
If u cannot take the fun of bargain, then just keep the item and dun post it in Buy & Sell lor.