These fish should not be sold as pets.
I completely agree with you on this, no question. [And I am fine with most of what you have posted [#14], some of what I previously said was misunderstood, but we move on.]
There was a movement in the UK to have a ban on these "monster" fish, without some sort of license, but certainly stores would not be able to just stock them and sell to whomever. There are many fish that fall in this category...some not so obvious at first, like clown loaches. I don't know what happened to this initiative.
In fairness to the store, however, it is possible that this lone pacu was simply caught with the SD in South America and made its way with the hundreds of fish to the exporter, importer, store. And in the latter, chances are less observant/knowledgeable staff would not even notice it. This happens with wild caught fish. I have spotted rare pencilfish in shipments of rummy nose tetra, and the false green neon in with cardinals, etc., now and then.
Byron.