Should I accept 8,000 dollars before tax on this used car?

Yesterday we went into a dealership and the salesman insisted that he bought the car for 8,500. He showed us a sheet from kelly Blue book saying its WORTH is 8,500. I realized this and starting arguing with the salesman. Me being only 15, he said things such as, "Why am I arguing with a stupid 15 year old?" Then I told him to show us the invoice because he has nothing to hide since they bought it for 8,500, anyway. He said he'd go bankrupt if they showed all the customers the invoice. So we left using the walk out technique. He goes and tells the manager that we are leaving, and we saw that didn't want us walking out (we have good credit). However, he let us go. Today, though, the salesman calls and offers 8,000 for the car. That's 500 below what he "Put into it". Obviously, there is still some room to negotiate. Any thoughts?