Methods for selling used bike
Best methods to sell a bike will vary depending on where you are located.

If you are in North America local Facebook marketplaces oriented to bikes or bike gear can be good. See https://www.facebook.com/groups/dcusedicyclemarketplace/ as an example.

Pinkbike has a buying and selling section https://www.pinkbike.com/buysell/ but it seems to be dominated for really high end bikes (think > US$5K).

The general rule is that if you can't sell something you are asking to much for it. Try checking Bicycle Blue Book https://bicyclebluebook.com to get an idea of how much your bike is worth. Many folks will say BBB values are BS, and I partially agree as values are dependent on many factors, but at least you'll get an idea what you should be asking ($100s vs $1000s).

Post Christmas winter is probably a bad time to try to sell a bike. Wait until the warmer spring weather appears and you may have more luck.