Tourists crossing into Niagara Falls is not "essential travel". Nor is people visiting relatives on the Canadian side. But that's been going on for weeks now. Pretty sure this whole "essential travel" thing went out the window a few weeks ago. Your guess is as good as mine whether the Government website has been updated. Personally, I'd make a phone call.

Worth noting is that the "seller" cannot bring the car into Canada. That must be done by the buyer. The same applies to selling a car in the U.S. - the buyer has to physically bring it in. The exception is using a licensed carrier.

This site spells out just about everything except "who" has to bring the vehicle in.

https://www.riv.ca/USCustoms.aspx