There are many things that should be done with that mountain bike of yours when it comes time to sell it. Your bike may have some visible wear and tear on it; some things can be replaced while the other things that cannot be taken care of should be brought to the attention of buyers. You can search more information about city e-bike via https://www.speed-e.ch/city-e-bike/.
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Replace any parts if necessary:
Before you start cleaning your bike you should first look over your bike for each section that looks like they need to be replaced. Is the chain rings and cassette ready to move out for a new one? Also see the cable, bar grips, saddles, and anything else you want to find a new brand if you buy yourself a mountain bike pre-owned.
Clean your bike:
First remove both your wheels from your bike as this will make it easier to clean those hard to reach areas. Once you do this, take an old cloth and began to wipe the entire frame with cloth and you may have to get a wet cloth to make this easier.
Make sure all moving parts work:
After your bike and wheel cleaner, put the rear wheels and it is time to clean the chain. If you happen to have a Garden Tool Chain Scrubber degreaser and have any that will be useful to effectively clean the chain.
Some last thing I can think of:
Be sure to check the wheels to see if they are true and anything else you might think. Also, you may want to pump up the tire with the correct tire pressure and make sure the saddle pointing straight down.