I sold a couple on Horsequest a few years ago, they both sold for decent money too and I do think the more serious buyers are on there.
However, now we have Facebook and all the many pages on there offering horses and ponies, it's worth a try as it's all free.
The market this year is dire though, apparently in Beeston sales last week Jack Russell pups were selling for more than many horses

(I've never been there, and would never personally sell mine in a sale, it's just what I've heard on horse forums).