First, it is very wise to assemble all of its records so that you can hand them over to the new owner. Registration is important to some prospective buyers while others couldn't care less. Its medical records will likely need to be accessible to the new owners. No matter what your horse's history at the veterinarian, it is always best for it if you are completely up front and candid about any past problems. Before confirmation of the sale, your horse will be vetted, most likely by a different vet than yours, but any information you can provide will save trouble for you and the new owner.
Most new owners welcome any tips on how to keep their new charge healthy.
For example, if you have used a coat or hoof supplement, tell the new owner. On the day that you show it to a prospective new owner, make sure you groom the horse meticulously from head to heel. A good curry followed by a smooth finishing with a soft brush is critical. Wipe its eyes, nostrils and ears, and make sure there are no shavings in the tail. Clean its hoofs as carefully and brush on some hoof polish.