When it comes to selling bikes, patience is key.
My thumb in the air measure is usually half the RRP if in good working order and condition. My last bike netted £400, not bad for a bike that originally cost me £799 but it took a good 3 months to shift. Of course, as standards and tech move on, obsolete setups will naturally be worth much less.
Popular brands will command a higher price, particularly winners various best bike of 2018 awards etc, niche brands are much harder to shift, but that's necessarily a deal breaker, if enthusiasts want what you have, eventually someone will make the right offer. For instance I have never had a problem selling a Surly bike, in fact as I am careful about getting a good price on used items, most often I usually make a bit of profit when it comes to selling. Discount retailer bikes are never going to hold their value, so avoid those at all costs.
Best time of year to sell seems to be spring when people are looking to start a new hobby or get out more as winter weather passes, so unless you have the latest model and stand to lose more to depreciation, maybe consider listing in Spring. I had a lot of low ball offers before I had someone interested enough to come for a look. Even my buyer tried to haggle me down, but I stood my ground and asked for the full £400 and he eventually obliged. Don't fear people walking away from a deal. A quick sale will never result in top price.
My personal preference is to list on gumtree which is free, and if I really struggle to drum up interest, I resort to
ebay which has a higher viewership, but that naturally comes with steep fees, but that when the priority is a quick sale.