So imagine you have a tank of fish that are doing great. You add a pre-quarantine fish and all of a sudden everyone in the tank gets ick. WHY would that happen? The introduction of the new fish caused enough stress on your old fish that their immune systems ability to fight off parasites decreased. So my point, unless all your current fish have been quarantined there is still disease in your aquarium and it is pointless to spend extra and buy a pre-quarantined fish. Do the QT yourself to be sure
I never did before but if your previous/current fish are clear of parasites and you want to add more new fish from those vendors, sounds like a solid plan. Meaning is your tank clear of ich currently?