Gumtree and facebook classifieds are incredibly full of time wasting arseholes, and functionally illiterate people. But they’re also the home of genuine buyers who’ll pay what it’s worth. Basically if you can deal with all the arseholes it’s still worth it.
Ebay cuts the timewasters but you’re in a big market so getting the best price is harder.
IMO it’s really important to have a good description, especially with spares/repairs- people need to know what they’re getting, if you leave them guessing then you’ll get a guessing price. Bizarrely my old Focus got more attention as spares/repairs once the clutch went, than it did when it was a fully MOT’d driver- because people were searching for spares/repairs I guess.
As uew-r says it also depends a lot on the actual car- some have specific value.