When buying a used car, how much can you haggle the price?

Lets say I want to buy a used car that is priced at 10k, whos fair kbb value is anywhere from 9k to 13k. Whats the lowest I could realistically offer for the car? 7k? 8k? I know it majorly depends on the seller and how badly they need to sell the car, but what do you think?