The best way to get around paying duty when getting stuff shipped into Canada is to ask the seller to declare the item as a "gift", with a value of less than $50CDN. The government doesn't apply taxes/duty to items that meet these criteria.
I've done this many times, and it works well. Of course, there are some things to consider. Obviously, if you're ordering something really big (like a carbon fiber hood, an engine, or a set of rims), customs may not believe the item is only worth $50 and they may investigate (there's very little likelyhood of this, but there is a chance). I've also found that some of the big-name places (especially on-line vendors) like Tire Rack or Summit Racing won't do it.
The biggest risk comes if an item is lost or damaged due to shipping. If you've ordered something that is worth $300US, but the seller declared it as a gift valued at $50CDN, then if you put a claim in to the shipping company (assuming you bought insurance) then all you're getting is $50! This is assuming that you've bought shipping insurance. Some of the big-name places automatically include it in their shipping cost (which is why they won't declare items for a value less than actually are), but most places don't so ultimately it's still a risk you take anyway.
I've done this with items from eBay MANY times, and it's worked without a problem.
Hope that helps.