well it works like this. some places will give you a great trade in value, then reamn you on the price of the car you are buying. other places will give you a nasty trade in price, but may play ball with the car they are selling you. either way you still get screwed and for the dealer their change over remains the same. it's just a different way of spinning it.
sell the car, then buy the car you want. much better option.