Be careful with certain websites, I have heard many horror stories of people who have been tricked by dodgy dealers into selling their horses for them to make money off of..
I find the best way to sell your horse is to put up leaflets in local tack and feed shops etc. Also make sure all horsey friends and instructors know that you are selling your horse and to spread the word! You may also want to tell your farrier about it as they will visit alot of people who may want to buy horses..
Hope this helps!
Good luck in finding a 5* home! 