You'll never get what you could get on your own at a dealer. The purpose of being in business is to make money. They have to buy your car at a wholesale, so they can re-sell it and make a profit. As businessmen, they are going to try to maximize that profit however they can get away with it. It's to be expected. I fought it for forever, but it just ain't worth it. When I bought my new Patriot last weekend, I took a completely different approach, and it saved a lot of trouble and headache for both me, and the salesman.
I just told the guy look, here's my trade, here's what I can pay a month, and here's how long I'm willing to pay it. I don't care how you work it out, but work it out, and I'll buy the car from you right now. And he did. Easy as that. I came out about $2300 negative on my trade, but who cares. With the rebates and other discounts, and him working to get me the right interest rate, I got the car I wanted, for the payment I wanted, for the term I wanted.
Two factors that helped me out are a) I wasn't asking for the moon, I was looking for a good deal, but I was actually pretty reasonable. b) I plan to drive the Pat for at least ten years. If I was planning to trade it in 2-3 it wouldn't turn out to be as good a deal, but like I said, the plan is to keep it.
The last car I bought was my XJ. I went in with an attitude about what kind of deal I was going to get, and in retrospect I wasn't all that nice about it. I got to the dealership at 1pm, fought with them all day long, lost valuable time out of my life I'll never get back, and stressed myself half to death. Finally left with my XJ at 1:30 am. Twelve and a half hours to buy a car, none of it enjoyable. By contrast, the "here's what I've got, here's what I want, work it out and I'll buy right now, don't and I won't" approach, coupled with a friendly attitude, had me spend less than two hours total at the dealer, and got me exactly what I wanted. Focusing and the payment and term I wanted, got me a great deal with low stress, and minimal time out of my life invested. If I had been hung up on trade-in, I would be hating life right now. With the rebates, and other discounts, the neg. trade was really non-factor at the end of the day.
I'm not saying you're wrong, or that this will work for you, just that it worked great for me.