A common marketing strategy used by Bay Area listing agents is to expose a listing to the market and then wait a week or two before listening to offers. This creates an opportunity for multiple offers to develop if the property is priced right and the inventory is low relative to demand. Rather than wait for the designated offer date, the above buyer wrote an offer early and requested that it be presented to the sellers right away. The sellers went ahead and heard the offer ahead of schedule. The listing was sold before anyone else had a chance to compete.
Sellers can hear offers whenever they want.
However, most sellers who are intent on waiting to hear offers won't be inclined to change their game plan. For a seller to forego the benefit of market exposure, your offer will have to be outstanding. The offer in the example above was for all cash with no financing contingency and the buyers offered to pay considerably over the asking price. You don't see more buyers circumventing the competition this way because most buyers feel uncomfortable offering over the asking price unless there are competing offers. They prefer to wait and see how many offers materialize before they decide how much to offer, or whether to offer at all.