This forum has a for sale section when you have been a member for a period of time, which is pretty reasonable imho.
The problem with selling a bike is that there is often a difference between what the owner wants and what buyers are willing to pay. I know this from my own bitter experience.
This time of year is not a great time to sell a bike, you will have more success in spring.
If you want a quick sale Gumtree would be my suggestion, just because
Ebay is rubbish .. but you would need to be aggressively priced