Here's what I know about these things, which is not a lot.
First, I was slightly wrong about Quebec TVQ, apparently the rebate form I had is in case you actually register in Quebec, if you leave within 10 days and register elsewhere, you apply for the tax back. A friend told me if I buy in Montreal and get a 10-day trip permit to drive home to Ottawa, I don't pay TVQ since I haven't actually registered for more than 10 days. I assume Ontario is the same.
GST in general (not specifically for cars) is not actually paid by businesses (just like VAT). You pay GST on your purchases and collect it on your sales, and send in the difference. Or get a refund if you paid more than you collect. You also don't charge GST on exports.
I don't know what special documentation would be needed, but if you're buying vehicles and having them shipped out of the country by the seller, I would imagine the GST wouldn't even be collected (that's how GST works for other exports). If you're buying them, gathering them up and shipping them yourself, you might have to pay GST to the seller (since he's not exporting them himself) and then claim it back, either by becoming GST registrant business who's exporting them, or directly from the government.
I'm sure this is a specialized area wrt cars, and I also know that there were a mini-scandal a few years ago when people were selling each other cars for the GST, then exporting them to the US to get the GST back, but never actually handling the cars at all. So it would be subject to more than the usual scrutiny I suspect.
Finally, check out this part of the CRA (Canada Revenue Agency, formerly called CCRA and before that Revenue Canada) site:
http://www.cra-arc.gc.ca/tax/business/topics/gsthst/taxable/import/ex_good-e.htm l (you can work your way up the tree if you wish to learn more about GST, and of course it's also all in French if you prefer that). [edited: the link I just gave suggests that a car dealer would _not_ charge you GST if you said you were exporting and provided evidence]
=andrew
(Message edited by awebber on October 16, 2004)