I'm sorry but I'm not going to buy a £50 lens from anybody only to then spend £150 on having it serviced. It's more efficient to use some judgement and buy a good lens in the first place. It would only work with a very expensive lens, and if you've bought a very expensive lens it will have a warranty won't it, so if there's something wrong with it get the dealer to fix it?
As for the idea that if you don't have your Leica serviced there's sure to be something insidious going on inside that will bite you is internet paranoia. There was a time when Leica M's were used as professional cameras shooting far, far more film between servicing than many here are likely to get through in a lifetime. But mirrors can delaminate the day after the camera is serviced, the shutter cloth could tear on the same day, in other words if it's going to happen it will happen anyway. So a regular CLA service does nothing at all for reliability, it doesn't stop the mechanical things breaking or you knocking the rangefinder out of adjustment. Once you've removed the concept of reliability from a CLA you'll then see a CLA only needs doing when an adjustment is needed, and a camera that's gone ten years may well go another ten without needing a CLA, especially when todays photographers would have a fit at the thought of shooting ten rolls a day.
The solution is simple, if it ain't broke don't fix it, and if a Leica noob have the seller pay for a CLA or give a warranty before you buy it because 9 times out of 10 the revelation of what a CLA can achieve in accuracy and smoothness is only because you've bought a dog in the first place.