I've seen both T. radix (plains garter) and T. sirtalis parietalis (red sided garter snake) listed as "northern garter" before. In most cases they were WC from Canada. In one case they were actually adult T- "Iowa" albino plains garters.

I've contacted the ad posters before, once when it was plains garters being offered. They were like "oh, thanks for the information" but they just continue to place ads for "northern garter" snakes. I quit even bothering to contact the ad posters anymore. It doesn't do any good.

Most of them don't give a darn and seem to list any garter snake they offer as "northern garter" regardless of what species or subspecies it is. If it's anything but a checkered garter, they seem to just list it as "northern".