My wife has been ripped off by a car dealer. What can I do?

My wife traded a 2015 Mitsubishi Outlander with 50,000 km on the clock and I believe that the dealer ripped her off. He gave her a damaged 2004 BMW 3-Series E90 with 167,000 km on the clock and $5000 cash. He is trying to tell me that the BMW is value is $10,000 and what I find funny is the day my wife got it home I rang up to cancel the contract, but was told that there is no cooling off period and I said there was and then I was told that the car is already sold.