Can't buy zeta replica?

You need to have actually built the original zodiac zeta to buy a replica of it. If you haven't actually built any of the weapons the option will not be there. And after doing so it will only provide the option to buy replicas of weapons you have actually built.

I think the issue is that you can't RESUME a replica if you never finished the quest to begin with. But if you turned in a Zeta, you clearly had to have finished the Zeta at some point.