for private sales i agree.
i mean I always post 10% above my bottom price and write OBO.
but for vendors, it all has to do with their relationship with the supplier and MAP (minimum advertised price).
The supplier does this for their authorized resellers, and many times gives them a back end rebate that they only get after they sell the goods.
if they violate the MAP, they do not receive that $ on the back end.
this makes it so only authorized resellers can compete easily, and the supplier can keep a closer eye on their products sales channel.
this is why on amazon or bestbuy.com you see "add to cart to see final price"