Let's say you want to buy something online. It could be anything: a car, freelancing services, a house, etc.
The seller doesn't want to advertise a price because it might be well under or well over what a buyer would be prepared to pay. If the seller just puts the car up for sale 'by negotiation', then the buyer doesn't want to just make an offer because they might offer to pay too much. This creates a 'Mexican standoff' neither the buyer or seller wants to show their hand by indicating the price they are willing to buy/sell. DealEscrow.com is a website that solves this problem. It provides a fair starting point for negotiations.