Hardly an expert but I bought my car from Canada and imported it to the U.S. couple years ago. The short answer to your question is, yes, I believe the current owner does not need to register it in Canada before he sells to you. We've got a bunch of Canadian members here. Hopefully one of them see's this and will chime in...
What I did prior to my purchase is ask my local DMV what documents I would need to register the car in my State. Here is what they said I needed:
1. Copy of 'Personal Property Registry Search Results Report' to prove there are no liens recorded on the vehicle
2. Current Registration
3. Title from previous owner (Canada does not Title vehicles like we do in the U.S.)
4. Bill of sale
Of course your State may have different requirements... Long story short, the guy I bought my car from did end up Registering the car in Canada before I bought it but I was told he didn't need to. In fact, the seller sold another car that he bought from the original owner. He purposely did not Title that car before selling it back to someone in the U.S. so the 3rd owner would actually show as the second owner, thus preserving its value as a low mile car one owner car. Hope this helps.