in a vibe, the ECM and the PCM are the same box PCM is powertrain control unit. ECU could be electrical, electronic, engine control,computer unit. basically the computer than controls the engine and if equipped, the auto tranny, and probably some other things.
I don't know what the differences are between the various part numbers listed.
I have read conflicting info about flahing the things. Some people say they can't be flashed, that is why when GM has a TSB that includes a new ECU/PCM program, they replace the entire ECU/PCM. A few matrix folks have said they have had theirs flashed. Maybe the things can be flashed, and GM decided it was more cost effective to treat the thing as a replaceable unit than to buyToyota ECU/PCM flashing equipment and training for all their dealers. That might be the difference in the various models in the ebay listing, different versions of code, but if so, then it might be worth finding out what the differences are, and which version of the program comes on the one for sale.