Afaik there is an active aftermarket for .ca domains. Indeed, trustee services are at least not welcomed. However, one can always incorporate a business in Canada (I've seen such services online, almost instant, and you will have a company named like "12345 Ontario LLC" as the result. Since the company would not be actively trading, as it would in fact be just IP holding company, related accounting should be easy.
I'm not a lawyer or accountant, so the above is not a legal advice.
Another issue to be checked: CA-based endusers would expect to pay for their purchases in Canadian Dollars using local payment methods. So, non-CA based domain seller should better provide this in one way or another...