Have you tried advertising (free) in all of these? www.tutti.ch www.olx.ch www.anibis.ch www.gratis-inserate.ch
In each ad, upload the maximum number of photos permitted (usually 3 or 5, I think), and make sure those photos are clear and appealing. No incomprehensible jumble of cellar-shelving behind the bicycle, but outdoors on the lawn, preferably in the sun or with good lighting, and taken from several different angles, including close-ups of the main features: gears, brakes, saddle, etc.
If there are any noticible scraches or other places of damage, photograph them, too, close-up, and declare them honestly.
Also, could you be more specific about the "minor tuning"? (= needs a good clean, or brakes need adjusting, or needs new gear components, or light cable needs replacing, etc.)
Make sure your ad is considerably more detailed that what you've written here. Check the maximum text you're allowed in the ad, and utilise it.
Include, for example, a link to this kind of bike (like people do when they are selling IKEA furniture) and info on ist specific purpose, and even to a positive cycling review.
Also the frame size and wheel size, and "suitable for a person x to y cm tall".
Also, some cycling shops / dealers also trade second-hand bikes on behalf of sellers, but unfortunately I know nothing of how this works.