The reason this is bs because a lot of stuff is pre-used, therefore not a commercial purchase so import taxes don't apply
In Canada, and many other places, there is no such exception. You are responsible to pay taxes on everything you purchase, period. A "trade" can be even worse because then customs would prefer to use the fair commercial value of the goods for tax evaluation instead of what you may have actually "paid." Here they also have the right to apply the fair market value if you significantly underpaid for the goods or received them for free. Only when importing however. Bought within the country, with few exceptions it's the value based on selling price. But not in the case of automobiles, then it's book value if you underpaid.
Do you want a scarier and more egregious example? For Canada, if you re-import something that already belongs to you with any alterations that materially affect its value, you are responsible for paying tax on the value of the whole, not simply the upgrades/enhancements you've made. There are ways around this, but they are not automatic, you have to go through a process. Example: You have a car with a market value of $20000 here in Canada. You take it to the US and have the engine stripped down, modified and rebuilt. Let's say that cost you $5000. When you come back to Canada, the value for tax assessment is that of the upgraded car, perhaps near $25000, not the $5000 you spent on the engine upgrade.
I got caught with this one myself. I had a nice diamond already in my hands here in Canada on which I paid taxes when I bought it. Let's call it an even $5000. I sent it to the US to get set in a ring because that special ring I wanted was not available here. The basic ring cost $1500 and the setting service was free. I had it sent back to me FedEx and of course insured for $6500 but with the customs paperwork clearly filled out describing what had been done and the Canadian origin of the diamond. I was taxed on the full amount of $6500. I ended up having to go through multiple phone calls with customs and I finally did get a refund on the portion of taxes of the diamond, but only because I could prove that I had no alternative to get the setting here in Canada.
In Canada, with few exceptions, everything is taxed. And even in the case where something doesn't "appear" to be taxed, it very well may be. Electricity is taxed, natural gas/heat, tap water, water you flush, rain and even the air you breath. Though most people don't get an itemized bill for the last two.