Check for leaks, and general appearance, if it at least looks good you know the owner made an attempt to keep her up. But for sure check wheel and suspension bearings for play. Look for bent rims and bars, make sure all the controls operate freely and that the transmission shifts thru all the gears. Check air filter and oil for cleanliness, look over the frame for cracks, look for wear on the footpegs, sprockets and chain. Before he starts it for you check to see if the engine is warm. Sometimes if the bike is a cold blooded or won’t run well cold, he may get it warmed up before you Show up so he can say, look it’s a one kick bike..... If you find Anything up with the bike you can try to bargain him down. that KDX is a great trail bike, not so hot on the track though. Welcome to DBA.