Well, you could start by looking at Carmax or Hertz Car Sales, both places that do not negotiate prices on used cars. That would give you an idea if your dealer is asking going rate, or more or less than. That problem is with a used car, you never are going to exactly what the dealer paid for a used car, so unlike a new car, you can't tell how much room you have to negotiate.
For example:
My son's girlfriend 4 weeks ago sold a car to a dealer for $3,000. It needed $2,400 in repairs (at an independent mechanic)
That car has been fixed and is listed for sale for $6,995. Now, I suspect the dealer did the repairs for less than $2,400, but assuming he didn't, he is into that car for $5,400, and his markup on the lot is almost $1,600. I think that might be ballpark for many used cars. So I would never hesitate to start negotiating at about $1,500 off asking price on a used car, and be happy with $1,000 to $1,200 off. I consider $300 to $500 to be a reasonable profit for a dealer to make.
And remember, new car dealers make most of their profit on repairs, then on used cars. They make their least profit on new cars sales.