hi
the ECU you are looking at on ebay is un-tunable, that is, you cannot edit the tune, update it, change it etc
it is a once only tune and you are stuck with whatever settings are on there
ie its junk and you cant use it
options such as the powerfc, vipec, motec, autronic, nistune etc let you tune the car as you want
so you can buy it, put it in your car, have your tuner "tune" it and set it up to suit your car
remaps (which is what is in the ebay ad) are basically a flash edit of the standard ECU
but the problem is no one can update the flash tune, so its basically read-only
only select few could edit that map, and it wouldn't be worth it
for a GTR, the powerfc is prefect and well worth every $
and you can resell them for the same price you basically but it for
for info on the powerfc check my signature for the PowerFC FAQ