I always find it's better to sell my own car first, even if it means buying a banger to ride round in for a few weeks. If you are looking at a used car, then you can negotiate hard with cash.
When I bought the GT, I bid them £1500 less than the asking price, telling them that's all I could afford. I said, you've got the car, I've got the money, lets have a deal. After a lot of faffing and pretending to talk to the sales manager, he came back and shook my hand.