Donedeal has been the best for me when selling bikes.Facebook Marketplace, post on all of the bike buy/sell pages and groups, Gumtree, DoneDeal, eBay, classifieds on here etc.
Get it posted in as many places as possible. It only takes 1 person that wants it to see it.
Put up a decent advert with as many details as possible, good clear pictures of the bike that actually show the bike and it’s current condition as opposed to a pic of the mountains with a small view of the bike in the foreground etc. All of this “PM for details” nonsense is nothing more than a waste of time. If it’s not advertised right, it’ll put genuine buyers off.