In an AC power grid, which is what all city scale power grids are these days, there is NO net movement of electrons in the wire. The electron that just went into your "light bulb" will pop out the same wire (not the other) a moment latter without actually reaching the light emitting part. Thus accounting for where the electrons come from is complete nonsense.
The field travels at nearly the speed of light, but there is no movement there.
That is beside the point. THe advantage of trading energy credits with your neighbors is that you don't have to obey the monopoly selling and monopsony purchase price that a traditional electric company dictates.
The field travels at nearly the speed of light, but there is no movement there.