I use
Horsequest if expensive (!)
horsemart if cheaper (!)
H&H
British Dressage for dressage pony
British Connemara Pony society (for Connemara pony .....!)
Things that restrict a horse sale is over ambitious pricing, remote location or "not novice ride" other than that, pick one of the bigger websites and go for it!
I've always had lots of success from Horsemart - but you do have to be realistic about price at the moment. Advertised one recently - Friday 4.30 p.m., was sold by 6.30 p.m. and I delivered him 8.30 a.m. next morning!
Others are adhorse, horsequest, horsedeals, newrider, horsehunter, The Horse Exchange, Adtrader and H&H of course.