Used car haggling

Hard to say. I have bought cars from both dealers and "off the street". If you are looking to pay cash I would just by off the street. If you are looking for payments, just expect the dealer to come down some. You do have the upper hand.
My story: Last year my son was looking for a new Challenger R/T. As we drove around the dealers we found a dealer that had 3 used ones. 2 were 09 and one 08. The 08 was classified new as it had never been sold, low vin number and $50K. 1 09 was 43K (more options) and the other was 40K with nice aftermarket stuff. So the haggling started. My son had a 01 Blazer to trade. We sat down with the sales guy. I told him that $40K with 6K on the odomater was to much for a used car, I "acted" like I did not want my son to buy it. The sales guy called the new car dealer and ask what the price of a new 1 with the same options. When he hung up he said that it would be $46 new. I told him that was to fu_king much for a new one. He ask what we would be willing to pay. I told him we would not pay more the $32K and payments for 5 years. He left the room to have a chat with manager or a cup of coffee. I did not think they would come down that much. When he returned he said DEAL. Of course they gave us 12K for trade in for a truck that was worth 4K. I know, that hurt NADA and Kelly BB.
Bottom line is if a dealer wants to sell a car they will work with you. Not all are bad but they do need to make money as well.