Wendy,
Two kinds of taxes for car you have to pay when you register the used car:
1. Buy from Dealer: GST plus PST: 15%.
2. Buy from Private: Just one of them(can't remember GST or PST), so half rate you need to pay.
No matter which one, the tax is upto the sale price or published whole sale price, which is greater. For example, if you buy for $10,000, and published whole sale is $8,000. You will pay tax on $10,000. If you buy it for $7,000, then you have to pay tax on $8,000.
The published Whole sale price for used car, you can find it in any small store for around $5.
Hope it is helpful.
Thanks