I am often astonished at the prices some cameras reach on eBay without any assurance of their mechanical condition. (Let alone a guarantee!) To be frank it sometimes makes me a little angry, as it means that someone is making an unfair profit.

To be fair, many sellers do their best to check the camera before selling it, but things like shutter speeds cannot be checked accurately without using special equipment.

Comparing the camera's meter against a known good lightmeter is completely pointless if the shutter speed is not correct, and in my experience shutters, which have many moving parts,  are much more likely to be incorrect than lightmeters, which have very few.

Dont' be taken in by the external apperance, this just means that the camera has not been used a lot, or kept in a case since bought. There are plenty of things which deteriorate with age, even without using the camera at all.

Items such as light-seals (or traps), were often made using the cheapest materials available, which also degrade very quickly! Other things, such as lubricants (which are often a complex mix of oils) also suffer from degradation with age. Grease, becomes a sticky wax, oils simply dry-out, and oxidise. Springs lose a little of their strength.

Even electronic components age, and their values change as they get old, leading to shifts in the calibration.

These factors combine to degrade the performance of the camera.

The exposures become incorrect, and uneven, or light leaks will fog your films, or the camera will jam. (With many models, eg, Pentax ME-Super jamming is almost inevitable!)

In order to restore the camera to a fully working condition, these items need to be addressed. This could be done in a piecemeal fashion, dealing with each as it becomes apparent, but would require several trips to the repair shop, and a lot of repeated work.

It is much better to deal with all these issues at one time, and perform a complete overhaul.

There are a number of issues addressed by the overhaul, and not all cameras will require attention in all these areas, but in general the areas which need attention are as follows.

Light seals (light traps) degrade and need to  be replaced using new (In most cases superior!) materials. This includes the internal seals often overlooked, as eventually they break-up, and particles get into the works, leading to jamming, they may become visible in the viewfinder, or partially obstruct the photocells in the metering circuit leading to incorrect exposures. 

Both curtains in focal plane shutters need to run at the same speed, to obtain even exposure accross the image. At high shutter speeds this becomes critical. This cannot be assured if the lubricating oil has dried out, or is contaminated. Also the springs driving the shutters can weaken (Especially if the shutter has been left cocked!), and this rarely happens equally for both curtains.

To remedy this the old oil needs to be washed away. This can often be done without dismantling the whole shutter, but not always. The shutter mechanism needs then to be carefully re-lubricated with special light oil. (Only tiny amounts are needed. Too much can spray about and get into places it ought not be!)

Only then can the curtian speeds be measured, and adjusted. (Note: Some camera service manuals state that the shutter cannot be adjusted. I've never read that manual, and not yet met theshutter that could not be adjusted with care. At least not one with all the pieces intact. )

Manual shutters are then adjusted for shutter speed, to as close the indicated value as possible for that camera type. Some cameras vary slightly in the precise timing of the shutter. This is quite normal. I have found that about half of the cameras I see need some adjustment. (I suspect that many weren't particularly accurately set at the factory!)

Electronic shutters are timed when the meter is calibrated.

 The trick with electronic design, is to arrange things so that exact values don't matter. (Many components are rather inaccurate to start with!)

However, many camera metering circuits do rely quite heavily on the exact value of at least some of the components, and a means of adjustment is provided. Some types of electronic components age noticeably over a period of years, and so the metering circuits need re-adjusting.

I calibrate the meter using a lens which I know to have correct apertures, and couples correctly to the camera, as this can also affect the metering. I measure the light falling on a white screen using a meter accurate to 1/10 stop. The meter is checked for a range of light levels, apertures, film and shutter speeds.

If there is any variation in accuracy with film speed, I optimise for 200 ISO, as this is now regarded as the 'standard' speed as opposed to 100 ISO which was the standard speed when I was a lad, and most of these cameras were new. 

The calibraton and adjustment of these items often results in a camera better calibrated than it was when it left the factory!

Focusing screens are dusted and if necesary, cleaned ultrasonically to remove stubborn deposits. This will not be done if the screen contains only a small number of specks as this operation is quite a delicate one, and the screens require careful handling and drying if they are not to be damaged by the process! (Here is a useful tip. Never go anywhere near a focusing screen with any solvent. At all. Ever.)

Mirrors are cleaned, ultrasonically if possible, or with an alcohol swab otherwise. This removes the thin film of dust which accumulates over time without damaging the delicate surface. Some deposits may not be removable, without risking further damage, but this is rare.

After reassembly the camera is checked for function and generally cleaned.

As a result of all this work, you can be certain that the camera will work properly.