It's legal to buy or sell any species of Ants in the US as long as there's a permit for transport. Legal to own P. Rhea but it's illegal to import or export without a permit for transport.
Yes, this is 100% correct. A reputable ant shop will tell you which states they have permits to sell to.
Does the seller have to apply for a new permit each time for the same species or just once? Does the permit for that specific species expire?
They need a different permit for each state they want to ship to.