They won't investigate anything, and I think the same way as 3.2TL, you don't pay taxes when you buy something on Kijiji, why should you do it for used cars? And the higher amount rule is just pure bullshit, I think that's the only time where you can pay taxes on money that never even changed hands. When you buy something on sale in a store (same idea really) you pay taxes on the discounted amount, even if it is below wholesale value. This whole used car tax system is just another cash grab.
The MTO bill of sale is used for tax reasons, a private bill of sale is for peace of mind.