I'm not quite a motorcycle expert, though I do have some experience working on them and do know how to drive..

Anyways, it's not much different than any other considerations on used vehicles. What condition is it in? How many miles does it have on it? Do the tires have good tread and no dryrot cracks? Do the tires have and hold proper air pressure? Are the brakes in good shape or worn out? Are the chain and sprockets in good shape?

Any used vehicle is gonna have some sort of issues, so the biggest questions of all are gonna be between your personal preference, overall condition, and price. Price factor not just the price of the bike but also the price of the consumable components such as tires and brakes and stuff as I mentioned..