Welcome to the board crep.
Start at the top, and work down.
This is what I look at when I'm buyin a bike.
Scratches on mirrors
Scratches on bar ends
scratches on cans or pegs
Look very closely for cracks on the plastic.......could tell you if the bikes been down.......check closely around all bolt holes, and screw holes for cracks.
Check front wheel for oily residue in case fork seals are shot.
Take a good look at rotors for scratching and pads for wear.
Check the oil to see how clean it is......ask him for service records or any bills he has for the bike.
Check tires for wear.......they're not real cheap if you have to replace......ask him when he last replaced them.....if it wasn't to many miles ago and the rear is extremely worn he may have been doing burnouts alot........hard on the rest of the bike too.
Grab the wheels/tires and shake them side to side to check for worn bearings too........never seen it yet but anything's possible.
Hope this helps.
Check chain for wear and slack.....ask him how often he adjusts it and if it's ever been replaced.......if it has low miles and it's been replaced you'll wanna know why.