Most anything sold in Canada, the warranty will not be honored if exported to the US. I would avoid Canadian trucks for one simple reason, SALT. Even just one year exposure to that much road salt will shorten the life of the truck significantly.
There are two factors that make the Oklahoma/Texas market a buyers market. As others have mentioned, oilfield trucks are numerous. Most are white so fairly easy to pick out. The other thing is that 50% of the people down there drive HD pickups. Farmers, ranchers, weekend rodeo pros...
I have bought used pickups from Texas before and would do it again without hesitation. Bought a used semi a couple months ago, went to MS to get it. $7000 cheaper, less miles, and not rusted all to heck than a twin from the same fleet being sold locally.