So much depends on what you're selling exactly. For a horse with a specific talent, the best place has to be the affiliations website - so BD, BS, BE, etc. Horsequest I have always throught of as being for slightly higher end animals - perhaps with a bit more emphasis on showing. Horsemart I do always look on when buying, but it's not always at the higher end of things I don't think. Horse Deals I can't seem to get my head round these days. Adhorse/Coloured Contacts for lower end/coloured types. Never had much luck myself with free sites or FB pages. For a quirky horse or one with issues, there's
www.projecthorses.co.uk - tend to get all ranges on there, from some real quality to much lower end of the market. Horse Gossip seems to have a big advertising section, but I've never actually bought/sold from there, so not sure how successful that is.