Red/White plastics makes it an 04 or 05, quick and easy way to tell the difference is to look at the parking brake lever if he's got one still, the 04 model will have a small horizontally acting lever you move with your index finger on the clutch perch. The 05 will have a large lever that you flip vertically. You can also tell from the 10th digit of the VIN, the # will be the year, so a 4 means it's a 2004.
Sounds like you're paying on the high side for this bike, judging from what you found on craig's list and what people normally pay for stock 660's out my way. However without knowing where you are, or having ANY useful details or pictures of the quad it's very hard for me to make a valuation on the machine from the internet. Check the frame for stress marks, cracks etc. Go over it with a fine tooth comb, and make sure it runs well. Take it for a ride if you haven't already. Make sure it starts up easily and quickly without any whirring or spinning noises, make sure it goes through the gears smoothly, make sure the clutch is in good shape by either giving it the beans and letting out the clutch or putting it in a high gear (4th or 5th) and letting out the clutch. If it moves forward a bit and then stalls, you're probably in ok shape there. While you're at it check the oil level with the dip stick (remember you don't thread in the dipstick when you check the oil) and see if he's been properly maintaining it. Take a look at the coolant reservoir under the rear fender and see if he's been keeping the coolant level inside the margins marked on the overflow reservoir. While you're back there pop off the seat and airbox lid if he has one, and check the air filter. See if it's clean or dirty, whether it's been oiled or is dry. You're hoping for clean and oiled.
If any of these things come out bad, use it as a position to talk him down in price. If it doesn't start easily, quickly and without any whirring and spinning noises you're looking at a new one-way starter clutch and gear, which is about a 300 dollar fix if you do it yourself and more if you have to pay someone else to do it. Use your best judgment on these things, it's your money and you're the one who has to be happy. You don't owe this guy a darn thing, and if you don't feel right about the purchase or if you think his price is too high and he won't come down on it then walk away. Your friend isn't paying for this, you are, so don't worry about him either. If he's a good friend he'll respect whatever decision you make and understand that it's not personal, it's business.
Good luck, and if you have any other questions I'm sure the guys here would be glad to help out.
