The Kelley Blue Book website evaluates the value of a used car based on condition, location, options, etc. Check it out. It also shows what cars are available and the asking price, mostly at dealers. Some in your area are asking up to about $14K for 2000 GTs with 50-60K miles. Private party sales should be less.
Yours doesn't seem to be a bad price if it is in great condition. If they haven't already had the timing belt changed, try to make that a condition of the sale. And, of course, check the oil change records and make sure the tires have plenty of use left. Whatever you do, don't let emotion rule. It's strictly a business decision no matter how great the car looks. Some time ago I was looking to get a good used RX-7. A young woman was selling a top-of-line model of the last of the first generation. It was in great shape, and I was willing to give her top dollar based on the retail price, but she wanted a couple grand more than that. I could not have insured it for enough to pay for it if someone totalled it on my way home. So, no sale. She must not have really wanted to sell it.