I would check NADA for a starting point. You can input hull specs, motor specs, and trailer specs seperately to get a value for each.
NADA Boat Page, click Here
From there I would take a very close look at the package, including a sea trial, and make specific notes about things that seem unusual or sub-par. A Marine Surveyor would also be a good idea, or even a trip with the boat to the local Triton dealer. I'm sure you will be getting the motor checked by a mechanic for compression so let the mechanic at the local Triton dealer do the check-up. He will be happy to charge you his hourly rate for a good once over of the boat and you will get piece of mind for a few hundred dollars.
And for what its worth, $6500 for a seven year old boat seems a little steep to me but not by much.