1. Make sure that the title is clear and actually matches the Bike numbers.
2. Make sure the bike has not been down, look for any small marks in the frame that would give it away. Things that should not be there.
3. Certain mods are ok, If they added a fork brace, changed the seat or added other items ok, But if the engine has been modded or opend up it is one I would avoid.
4. Like all bikes look at condition, how was it kept and stored. With the Max make sure that the normal maintence has been done, Look at the fluids, All the Fluids, Antifreeze, Rear diff oil, brake fluids front and rear. All of these should be changed on a regular basis and if they have not been you can bet the bike has also been neglegted in other ways as well.
5. TEST RIDE!! and shift it hard to see if it pops out of gear. The 1985 models and some others that have been riden hard will often have 2nd gear problems. (This is true of other bikes as well)
6. Then there is all the normal stuff you want to look for on any bike, fit and finish, leaks brakes, tires etc.
There are alot of good bikes out there for sale and if you are patient you can find what you want for the price and condition you want as well.