If you can get the seller to break it down look at the gas tube like the man said, but also look at the shut off valve. The corrosive ammo can affect the seal of this valve. There is a fix that you can do with a paperclip if necessary, but better to get one in good conidition.
I have a Yugo 59/66 in excellent condition, wow, what a bargain. It really is a nice rifle. Cheap to shoot, dead reliable and pretty accurate, 3" or so at 100 yards with handloads. Might be better, just haven't had time to work on loads lately.
Also, if you can get the owner to break it down, make sure that the barrelled action doesn't move in the stock particularly after the trigger assembly has been removed. There is not real bedding on this rifle. The action is just kind of wedged in place.