It’s up to you to make sure to do your due diligence when selling.
If the type of home or planned use for the horse is important to you, consider a home visit. This isn’t generally standard practice, but it’s your horse until you sign him over. Some sellers, especially rescue organizations, require new owners to sign contracts limiting what the horse can be used for and prohibiting breeding.
AQHA and other breed registries offer a check box on the registration papers notifying the organization that you’d like to be contacted as a potential home if your horse ever becomes unwanted.
If these things are important to you, make sure you’re up front with the buyer throughout the buying process and that everything is in writing before money and registration changes hands.