Registration is important to some prospective buyers, while others could care less. His 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 him if you are completely up front and candid about any past problems. All horsemen know that every horse has medical issues, just as every person does. The humane thing to do is to share this information so that your horse's new owner can maintain him in the comfortable manner he deserves.
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. After all, you have spent hours and days getting to know your animal, and there is no reason to let all of that valuable information go to waste. It might seem like minutia, but a new owner will appreciate any tips from you. But before he goes, you have some work to do. Aesthetics play a huge role in the show ring, and in a sale.