A private sale runs on trust, and paperwork is how that trust survives the journey home. Without a dealer or agent keeping the file straight, it is down to you to make sure the right documents exist, match the horse standing in front of you, and come away with you. Four groups of paper matter: the passport, proof of the sale itself, registration documents, and the health record.
The passport: check it against the horse
The passport is issued to the horse and must be handed over at the sale. Before money moves, open it and check the details against the animal: colour, sex, markings on the silhouette diagram, and above all the microchip number — a scanner reading at the yard settles any doubt. A passport that describes a different horse, or a seller who cannot produce one at all, is a reason to stop, not to negotiate. After the sale, the change of ownership needs to be registered with the passport-issuing organisation in your name; until that is done, the paper trail still points at the previous keeper. Also look at the food-chain declaration page, because a horse signed into the food chain carries treatment restrictions you should know about before your vet ever treats it.
A signed bill of sale
Even between friendly private parties, get the sale in writing. A proper bill of sale names the buyer and seller, identifies the horse by name, description and microchip number, states that the horse is sold, and is signed and kept by both sides. If anything was agreed verbally — the horse is sold as seen, or tack and rugs are included, or a trial period was promised — those words belong in that document, because memories of a handshake diverge fastest exactly when something goes wrong. Keep your copy with the passport, not in a phone photo library where it will sink under a thousand pictures of the horse himself.
Registration and health papers
If the horse is registered with a breed society or a competition body, ask for the registration certificate and whatever transfer form that society uses, signed by the seller while you are still standing in their yard — chasing signatures weeks later is miserable. On the health side, request the vaccination record (often inside the passport itself), the most recent dental record, and a summary of any significant vet history the seller holds: past colic episodes, lameness investigations, allergies. If you commissioned a pre-purchase vetting, the vetting certificate and report are yours and should be in the file too. Sellers who have genuinely cared for a horse usually produce these without being asked twice.
All of this paper describes one horse's life up to the moment he becomes yours, and KinsLe's Transferrable Profiles are built for exactly that handover — the horse's profile can move to you with his history, records and routines intact, so nothing the seller knew about him is lost at the gate.