TJ, like esshup said, the blue book value might tell you that you're getting more than a fair deal. If this is going to be an additional/part time atv so that everyone in the family has a ride, it might make a lot of sense to purchase it. If you still feel like it's a little too many miles for the money, ask for less!
BTW, I thought it was funny when said you were replacing a Bayou, I wouldn't have given you that advice based on my atv! Oh well...
