"bitcoin is a medium of exchange but not a unit of account." Very true,however from an economic point of view it is a terrible medium of exchange,you have large spread costs,a difficulty sourcing and transferring product,issues of divisibility and and a massive level of price instability relative to the currency being exchanged.

Your example of buyer and seller also makes that point indirectly unless the bit coins are instantly convertible to dollars someone is going to loose out in any price move.

The cheapest way to pay someone in dollars is to actually pay them in dollars.