I disagree and think that it's a decent deal if you can get the price down another $2000 or so. It's comparable to a same mileage US M3. I just imported my M3 at the end of September and it's a great deal to get it from there but the price difference in this case is marginal. Don't forget that if you brought it in yourself, you might have to pay for transport if the car is not close you, and you still need to pay for RIV fees, and provincial and federal inspections which adds another few hundred bucks, not to mention time spent. That said if you look hard you can probably find one in the States with more features such as NAV for the same price or less. I'm so glad I got iDrive because you can control so many of the car features from it, especially if you have the M drive button.
As far as warranty, US cars have warranty in Canada, they just don't have free service. You will need to go to a US dealer for that. However, as it's a 2008 model, the warranty and service agreement will run out shortly anyways. So that really isn't an issue. Just keep in mind that an oil change that you need to do once a year or so will cost you around $200-275 from a decent shop and almost $350 from a dealership. Mainly because the synthetic oil is rare and costs about $20 a liter and you need about 8.5L per change. You can buy this oil from the US as well but not many places stock it. It's an exclusive oil they use for all M cars and it's essentially a race quality oil.
A US car will be worth less than a Canadian car but you paid less to start with so it's not too bad. I've seen a similar vintage Canadian M3 sedan listed for $59,000 in Vancouver. I rather pay $48K for a US car. The only difference is the speedometer anyway. Looking at car in question the only major features it's missing is the NAV/iDrive, the upgraded stereo which someone people don't like anyways, and potentially the M button, though I'm not sure how often you would use it.
Should go and negotiate and see if you can get the price down.