The truth of it is that Arapaima gigas is listed under CITES Appendix II.
If you have on in the US, it needs to have paperwork showing it was taken from it's country of origin legally. If you do not have that documentation, it makes it blackmarket and illegal in ALL 50 states - whether that state prohibits them or not. The reason you need to have paperwork is to prove it was brought in legally. If US F/W officers show up one day and asks to see proof, they won't take "Trust me, I got it legally at a petshop" as an answer.
I would not buy one unless that company could provide the legal paperwork. LiveAquaria.com is NOT a good supplier for Arapaima. They provide misinformation about adult size, proper care and after talking with them myself, refuse to add the legal paperwork or pass on the name of their importer for confirmation that they are indeed legal animals.
MOST Southern US states have laws against them. Some local laws regulate them as well.