A couple of years ago, when the Canadian dollar was worth 87 cents American, it made a lot of sense to buy a used Miata off ebay US and import into Canada. Now that the dollar is at par, it makes even more sense. But there are few steps you have to go through before the red tape is all cut away, notwithstanding NAFTA.
In short, if you have some spare time, don't mind dealing with bureaucracy TOO much, and live reasonably close to the border, then it's a viable option. However, here are some steps you should follow:
1. Shell out the bucks to get an independent inspection done of the car before you buy, or inspect it yourself before you hand over the money, or put the money in escrow. Carfax means crap, the automobile could have been in accident and a claim not put in for insurance. Don't even bother bidding on a Miata with salvage papers, you won't get insurance on it in Canada and so it will only be good for parts or for racing on your Grandad's wheat farm. Do NOT be afraid to walk away from the sale if the car fails inspection.
2. After you have won the auction, now the fun starts. You have two border agencies to worry about, US and Canada. Essentially, US customs wants to make sure you are not stealing the car, and Canada customs wants to make sure you pay taxes (personally, I think this says something about the collective psyches of both countries). Unless it's a complete pain in the rear end to do so, I would visit the US customs house BEFORE you try to bring the car across and pick up the special form that you have to fill in before they stamp your papers. Say hi, be reasonably friendly. You will need to fill in the form and fax it in along with proof of ownership and give at least a couple of days to run a check on the vehicle.
Don't be a dumbass and show up with the car and try to get your papers stamped immediately. Don't be a BIGGER dumbass and drive across the border and not get your papers stamped at all, the CANADIAN border guards won't stamp your papers without the US stamp.
DON'T let the ebay seller offer to take care of the paperwork for you. Remember, it's your butt hanging in the wind if the US border guards don't clear the vehicle. Coincidentally, there are many storage areas near the border where brokerage houses will store your vehicle for X number of dollars a day while the paperwork clears.
3. In terms of time and hassle, the Canadian border guards are easier to deal with, they just want your money. You have to pay GST (5%) and a special import duty for foreign cars (6%) and unless the Miata is more than 15 years old, you have to pay special handling fee of a couple of hundred dollars or so. It is EXTREMELY IMPORTANT that you have documented proof of how much you paid for the vehicle, the border guards will want to see proof.
If you got a really good deal on the car, make sure your documentation is EXCELLENT because if they think you are lowballing the price to cheat on taxes, they will bust your butt and maybe say your have to pay taxes on the TRUE value of the car, which of course would be considerably higher than what you paid for. Whatever.
4. By the way, did I mention insurance? If you are paying a shipping company to drop the Miata off near the border, this is not such a big problem. If you are planning to drive the Miata across the US yourself after buying it, you buy your insurance IN CANADA, not in the US. You can buy 30 day insurance that will cover your car expressly for the purpose of driving it to Canada to be imported. That means if you want to detour to Disneyland before heading back to Saskatoon, you won't be covered.
Note that you will get a piece of paper saying you are covered for this 30 day period, you won't get plates. That means you have to drive the car without plates from site of sale to the border. You might want to inquire of the seller if they will let the plates stay on the car until you get to Canada so you don't stick out like a sore thumb on the American highways.
There is a hole in the insurance coverage. Technically, once you are on Canadian soil, you are no longer covered. So once you clear customs, you should hustle on to the next steps.
5. Once across the Canadian border, you need to get mechanically inspected for provincial certification. Before heading off to a registered mechanic who can certify the Miata for you, pull over to the highway and check the tread of your tires. If your tires are worn, you might as well go buy a set of new tires and get them installed before the inspection, because they always try to nail you on that. They will also as a matter of course want to install new brake pad and a parking brake. And if your headlights are burned out, you may as well head over to Canadian tire and replace them yourself, duh.
6. Okay, after the car has passed mechanical inspection, you may buy insurance for it and pay provincial sales tax (unless you are in a province where that tax was harmonized with the GST, in which case you paid for it at the border). You're home free, right? Er, not exactly. Even though you are at peace with the provincial bureaucrats, this wouldn't be Canadian unless we had overlapping provincial and federal jurisdictions, right?
7. In the next couple of weeks or so, you should get a letter from Transport Canada saying you need to get ANOTHER mechanical inspection done. Now you need to watch out for the gotchas. You can either a) Find a mechanic who specialize in fixing US Miatas to Canadian standards or B) Do some of the work yourself and save hundreds of dollars.
The two things you need done are to get running lights installed and your front bumper upgraded. The running lights kit can be bought from a dealer or Ebay for about a hundred bucks (or less). Plugging in the gear is pretty easy, I did it myself and I'm no mechanic. The front bumper upgrade is really a dumb thing to worry about. Apparently, Transport Canada thinks American Miata bumpers are different than Canadian Miata bumpers. Well, they sure as hell aren't different for second generation Miatas so if some Mazda dealer wants to charge you a $1000 bucks to swap bumpers, then go to a different dealership and just order the damn piece of plastic that is a Miata front bumper for about $200 and get the piece of paper. You can swap bumpers to follow the letter of the law, it takes about an hour but boy oh boy do you ever feel dumb when you pry off the old "American" bumper and its the EXACT same part as the new "Canadian" bumper. Whatever.
Canadian Tire will then inspect the car, which means for about $100 a mechanic will turn on the running lights and look at the piece of paper that says it has a Canadian pumper, and fax the appropriate forms to Transport Canada to get them off your back.
8. And baby, you are done.