Depends on the dealer and what you are buying. When I traded an old honda accord, it had been in a major accident, the dealer paid me more then I would get for it selling it privately. They did this because I was buying a new Outback. Doubt I would have gotten that deal buying something used.
Traded in my 2010 BMW R1200GSA about a month ago, they are selling it for about $1000 more then what they gave me on trade in. The price was good, no hassle dealing with people looking, plus the service it would have needed, brakes, etc.
Hard to say anything without knowing any details about the car and what you are wanting to buy.