If you are buying another car, your best course of action is to find the car you want to buy (but not buy it yet), find a buyer for your car privately and then find a dealer that will allow you to do a 'private sale' through their dealership (basically once you've find a buyer, you'll trade the car into the dealership and then the dealership will sell it to your buyer). The benefit of this is that you'll get a credit on the tax for the sale (trade-in) value of your car. I believe most dealerships will do this and will charge minimal fee (maybe $400 transaction fee). It used to be bad for buyers since they'd have to pay full taxes on it but now in Ontario, full taxes are charged on private sales anyway and if they do it at the dealership, they'll pay tax on the purchase price, if they do it privately, they pay tax on the purchase price or the red book price, whichever is higher.
If you can sell it privately through a dealer and get the tax credit on your purchase, you might not do so bad but you'll still probably take a significant hit unless you got a really good deal when you bought it.