Facebook is always a good place to go! Join a group for buying/selling horses and you will have people interested in hours. It’s also nice because you can get a glimpse of who you are selling your horse to!
Facebook is always good, but you can find many other websites and maybe ask your coach or yard owner to spread the word
Horse Quest and eHorses are both good sites that I’ve had positive experiences with :)
I have no clue but maybe search up a few ads for ponies in your area and put the ad on one of those