I wish the knife swap that vivi organized had worked, but a lot of people don't like reddit.
That said, there is an active knife swap on reddit. I've never used it, so I can't say how good or bad it is, but it is definitely active. Of course there is Bladeforums, which requires a paid membership. There used to be a knife forum where you could sell for a dollar commission. I never bought there but I sold a few. I'm not sure they are still around. Other than that, word of mouth i guess?