What actually happens a lot are ads for sale on Agon and the same item for auction on eBay. Like the new in box pair of B&W Nautilus 803 speakers from North Carolina going on now. The high bid is now more than the asking price on the Agon ad.
When a sale is over at a retailer the price goes back up, so I see no reason why anyone should be prevented from raising the price on a regular classified ad.