To be honest, the year model will help in determining the price to offer.
Things to look out for is the following:
Wheel bearings
Linkage bearings, if you pick the bike up at the rear just before it leaves the ground you will hear it make a clack noise, you will also feel it.
Forkseals
Maybe take the tank off and see how clean the bike is under the tank and check if the airfilter is clean. You need to clean it before every ride.
You can also take the pipe off and look into the barrel from the front, you can then turn the motor with the kickstarter till you see the piston and see if it as any scratches. If it does, it might be because of piston slap or a dirty motor.
This is what my bike looked like when I sold it for R27 000.00 (2007 KX250)