Why argue with $6500? Sounds like a good deal - I'd be surprised if they came down any. I wouldn't, if I was the dealer. But, you can always try.
Was the car dealer-serviced? Have you reviewed the records?
For comparison, I just bought a 78,000 mile 1990 LS400 for $7000 (after TTL) - one owner, well-cared for, and all Lexus records (all oil changes every 4000, regular mileage services).
My budget for my commuter was $5-6,000, but for what I got, I'll gladly pay the extra $1000.
As far asking for it to be lowered, just tell them that the car isn't worth that, but you'd love to buy it. Offer them $4500, let them laugh at you, and see if they counter - if not, they don't think you're serious. If they do counter, then you got a deal going. Then the choice is up to you - how much are you willing to pay for that particular car? Set this price in your head before the offers begin flying...