I would check -everything- before making an offer. Here is what David Odess suggests:
Camera body: Remove the lens and magazine. Press the release button, and hold it in. While you are holding it in, check to see that both of the rear flaps are opening all the way. Now let go of the release button, and check to make sure that both flaps have closed completely.
Lenses: Set the shutter speed ring to one second, and set the f stop ring to the largest opening (smallest number). Look into the front of the lens and fire the camera. The shutter blades should close down, then open instantaneously, stay open for one second, and then close again. While you are watching this happen, check to make sure that none of the shutter blades are broken or appear to be out of place. Also, notice how long the one-second exposure actually is. You can pretty much tell how long one second is. If the shutter is dragging at one second, it is an indication that the shutter needs to be cleaned and lubricated.
Wind the body again, turn your strobe on, and attach the synch cord to the lens. Set the shutter speed ring to the fastest speed and set the f stop ring to the largest opening (smallest number). Point the camera and the strobe to a light colored surface, and, while looking through the back of the camera body, fire the camera. You should see a brief, large round flash of light through the lens. If you see no flash of light at all through the lens, check to insure that the "VXM" lever on the lens is in the "X" position (only on the older C lenses). If you see part of the shutter blades instead of a round flash of light, there is a problem, and that lens should not be used. Check each of your lenses using this procedure. If you are photographing a wedding, it is a good idea to perform this particular check each time you change film magazines.
For older C lenses only: Set the shutter speed ring to one second, and set the f stop ring to the smallest setting (largest number), and, while looking into the front of the lens, press the depth of field preview lever on the lens. When you press it in, the diaphragm blades should quickly snap closed. If they close sluggishly, it is an indication the lens needs to be serviced.
Film magazine: Remove the lens from the body and put the magazine on. Remove the dark slide and try to remove the magazine from the body. It should not come off the body with the dark slide removed. Insert the dark slide in the magazine and try to fire the body. The body should not fire when the dark slide is inserted. Take the magazine off the body and remove the dark slide. Look at the serial number plate on the magazine and make sure that the light trap or light trap foil is not sticking out of the side of the magazine, into the image area.
A-12 magazines with the plastic flip up crank: Flip up the black plastic film advance crank and turn it clockwise. It should stop turning the moment the red/white indicator changes to all white. At this point, the number 1 will show in the film counter window. Fire and wind the camera 12 times while watching the film counter window and the red/white indicator window. Each time you fire the camera, the signal flag should change from white to red. When you wind the camera, the number in the film counter window should advance to the next number. After the 12th exposure, wind the camera and try to fire it again. It shouldn't fire after the 12th exposure. Now remove the insert from the magazine. The number wheel should return back to number 0.
Note: The newer A-12 magazines do not have a number 0. In this case, you will not see any number in the window.