counter offer to dealer for used car

I would say 10% - 15%, but it depends on the car and the price.

Look online for evaluations and price guides and offer what you think is reasonable. They can always say no.

In my experience, dealers will come down up to $1000 on a used car from asking price if you haggle. It all depends on the situation, how long the car was on the lot, if it's a popular car, how new it is, condition, miles, etc. Click on the Research link across the top of the webpage to get some pricing. Used cars are in demand right now, so getting a deal is getting harder to come by.