price negotiation in person is the way we do things at my dealership. What any dealer wants to see is that they have a committed buyer. I personally try to avoid talking about price over the phone. Why? You will simply take my number and go to my competitor and aske them to beat it. You can't sell a car over the phone. That said, a lot of high volume lots will negotiate over the phone.
You also have to consider if your offer is reasonable, see what kbb, nada, and edmunds have to say. Depending on the price of the vehicle, the tax could be $2500. Not every used car on the lot has $2500-4000 of mark up. You have to remember, that a certain point, if the vehicle has been on the lot a while the dealer will discount it to where they will break even. Just because the vehicle has been on the lot a long time does not mean the dealer is going to take more of a loss than they would at the auction house.
Also, the whole writing a check or paying cash to get a deal done does not work.