Hey Tasha!
Fully agree you've not got a Northern. Do you happen to have an image (or know) the pattern on the underside? If it's small amounts of black dots, with mainly pink/whiteish colour - it could be a Merauke. However, to me it's looking more like a Halmahera (which would have primarily dark colours underside, big areas of black/with splotches of pink patches).
But definitely need to keep your Bluey in humid conditions (60-80% if Merauke, 80-90% if Halmahera). Hope this gives an idea to start with!
Also - is the mouth ok? It looks a little crusty - it could be potential mouth issues, especially as they thought (s)he was a Northern (or possibly just some food if you've recently fed them!). I would keep a check on it

Depending on how many others were in the shop, it may be an idea to tell Kijiji they are not Northerns...
All the best!