In the past, before HST, if you buy a car not through car dealer, you only have to pay PST, but NOT base on the price you said you paid to seller, it is according to that BLACK book (???) which I think usually that book inflats the price (hence more tax). If you buy the car (as used car) through any car dealer (most likely trade-in), then you have to pay both GST and PST on the price you paid to dealer (Government trusts dealer). Only if you transfer your car to your spouse, then you don't have to pay. It seems that the law has changed that if you transfer your car to your son (or daughter), then you don't have to pay tax, and you don't have to do any Emission, Safety Tests...
Tax means money to UNIVERSAL health care (like in 2005, Liberal Mcguinty HEALTH tax!!!).
Maybe when Canada has two-tier health care system, (like US), then our tax can be lower (Tax in US is lower).