Usual suspects...Horsequest, Horse Select, H&H. If you want to advertise a pony then the same websites but also Pony Club Sites as they are regularly looked at & good ponies are sold quickly.
You need to advertise in the places your target market will be looking, PC site if suitable for a teenager, BE if eventing, Horsequest is good as has a lot of ads but they do move down fairly quickly. These are the sites I find best for competition types. Horsemart is not too bad but it seems to be less competition types more allrounders.