1) It's an ok price but not great. You say a 'junk' shop. I would either get a 14 day return, or have them knock the price down to $75. Unless it has the Yashinon lenses, where this is a good price if the rest of the camera is in usable shape.
2) DO NOT use the self timer. If you really must play with it, the flash sync MUST be set to X. Be prepared for it to grind to a halt. IF SET TO X, a light pressure on the lever should make it run out. Again, best to just ignore for now unless you KNOW it is critical for what you will be doing.
Seriously, DO NOT TRY THE SELF TIMER if the flash sync is set to M.
Open the back, put the shutter on B, open the aperture all the way, hold the shutter down while looking through the lens. You'll know if something is wrong- it should be clear.
Take a flash light to the front lens, glance it sideways, look for scratches, etc.
Try the shutter at different speeds. The odds are that it will be slow at slower speeds. This involves a cleaning, but if oyu don't use slow speeds, it won't be a problem. It's more of an indicator of age and dirt and lack of use.
Take an empty spool along. Put tape along one edge of the central core, and place this end at the left so the silver gear inside the take-up area rides on the tape (well, tape isn't required, just makes the counter work smoother). Close the back, wind the 'film.' It should stop at one. Push the center button on the wind knob, release it, then wind again. It should stop at 2.... keep going and test all numbers.
I prefer to bring an actual roll of film. I have some junkers for this. Load, wind to start, close and wind, etc. Except when I get to 12, I open the back up and look at where the film is positioned. There should be an inch or two still below the film gate. If not, there are some serious frame spacing issues.
OK, now for the viewfinder. See if it opens and closes easily. Make certain that there is a magnifier lens in the flip-up magnifier (the glass is removable). The odds are that the view will be a bit dirty and dim. Fixable with a cleaning and new mirror most times. When closed, the inner panel should sit evenly in the opening. Check for paint chips and wear- a good indicator of overall handling and care over its life.
Find an object at inifnity- 150 feet or more. See if the viewfinder shows it crisp, or if focus goes past infinity. Not a killer but an indicator of alignment problems.
Focus- go back and forth. Feel for bumps, jumps, binding, etc. Watch the front panel to see if it makes any twists or bends while focusing in and out.
Put the lens on infinity and look at where the back of the lens panel meets the body. This should be pretty even all around. It should not touch the main body before infinity, actually should stop just shy of the main body. If one side hits first while the other is still moving, alignment issue.
Check the mating between the main body and the back. That joint should be even and tight all around.
And then, how's it feel? You like its balance? How the shutter release acts? The viewfinder? Could you see using this camera day after day? A mint camera that doesn't feel right ain't worth anything to you if you want a user.
