I'm planning to buy a used Nikon F5.
I have no experience with a film camera and I want to know what all should one be careful of before buying a used film camera and more specifically a Nikon F5.
Nikon film cameras are practically indestructible so unless used heavily by a professional photographer, who mostly switched to digital five to ten years ago, it should appear to be in pristine condition.
The F5 is old now so you will probably need new batteries. Specific areas to check would be the mirror, film winders, and lens mount. There should be no sign of wear except maybe on the grips. If the camera looks old, tatty, or worn then just walk away and look for another. Lots of owners will also expect an unrealistic price because they were so expensive when new, shop around to get an idea of the right price.
You could also consider the the F80 if you want something lighter and cheaper or an F6 if you want the 'end of the line' camera.
You should test it. First, check its metering and compare its result with an external light meter. Then you should test the shutter. If you were buying from me, I'd offer you shoot a roll of film with it. Also take a photo with a long exposure time while with the lens cap on to know if camera's body is not healthy. Shake the camera and make sure it does not make any sound. Test the aperture diaphragm at high and low numbers. Test continuous mode. Inspect the lens mount. And finally, it's important to know the number of shutter actuations.
The first thing that failed in my film SLR was the rubber sealings of the back cover. Light seeping thru a bad sealing gets right onto film. At least that's the case with my Yashica FR1, that has a back cover wide as a barn doors. Otherwise my camera has only the normal shine of long use on the edges and button surfaces. So check the sealings.