Google recommends a few searches when I look for why they are cheaper, including "why are Quebec drivers so bad?" but I imagine it's the road salt they deal with a lot of the year.
It sounds like I'll have more room to negotiate than I thought. I've read that dealers will put a 20% markup on used cars, generally. So maybe I can negotiate their profit down a bit. The car's been sitting on their lot for three months, and the colour, 2WD and 1-generation old body style could help my case.