I think this is a nonsense 'Rule' and should be the sellers decision if he wants to post the price or accept offers, but I do have a solution which works really well on another forum I visit.
All negotiation are carried out open and visible on the thread and the communication only goes to PM when an agreement has been met and personal details are being exchanged. This way, anyone interested in buying the item can offer openly and the seller can then make all negotiations transparent.
See here for the full rules: [url="http://www.avforums.com/forums/view_trading_rules.htm"]http://www.avforums.com/forums/view_trading_rules.htm[/url] (They look heavy but in practice it's really simple!)
I do hate it when I post an item at a price and then get shot down by people who have either no interest in buying, ill informed on current values or are simply jealous. This then has an effect on potential purchasers who may have been willing to pay the amount advertised. I think these posters should have a 7 day ban until they learn to keep their hi-jacked messages out of the classified sections. If they feel really strong about it then they should start a new thread in the General section and away from the selling thread. Afterall, if they butted in like that when I'm negotiating a deal with someone in public then I'd probably knock them on their arse.