These are legal to own within Canada, but there are provincial and municipal regulations that may prohibit ownership. These are not super rare by any means, many hundred are sold throughout the country every year, I have a few friends who regularly breed them. They get extremely large, and not appropriate for housing with fish (however a friend of mine had two that were close to 3' long living in a 15,000 gallon fish tank with a bunch of large cichlids, catfish, pacus, and a 16' anaconda). They are however desirable, and supply is only limited to what is bred here, or smuggled in as importing turtles to Canada is illegal.
If your turtle is a fair size then $400 is a very reasonable price. I think a turtle with a 10-12" shell should be worth $400-$500. Smallest individuals 4"-6" should be about $150-$200… And anything in between that should be about $250-$300. These are fair market values for this species.
Good luck with your sale,
Justin Morash
East Coast Vivaria