It is a very good idea for both buyer and seller to have contracts in case something goes wrong. This is why it is wise to have a vet check or to bring along someone knowledgeable.
Here is a sample of a legal document for both sellers and buyers: The printable version is HERE

PURCHASE CONTRACT
Horse Name:_______________________
Breed:____________________________
Gender:___________________________
Foaling Date:_______________________
Color:_____________________________
Markings:__________________________
Registration #______________________
Suitability:_________________________
Temperament:______________________
Training:___________________________
Sale Price:________________________
Payment Method:___________________
SELLER INFO:
Full Name:_______________________
Street Address:____________________
City/Town:________________________
State/Province:____________________
Phone:___________________________
Email:____________________________
Known Defects/Vices in Horse:
___________________________________
___________________________________
___________________________________
PURCHASER INFO:
Full Name:________________________
Street Address:____________________
City/Town:________________________
State/Province:____________________
Phone:___________________________
Email:____________________________
Payment Term Conditions (if applicable):
I promise to pay $_____________ per month for the period of ___________months until said horse is paid in full.
The horse (circle option) will / will not be released to me until the full purchase price of the horse is paid. If I fail to do so, the horse may be repossessed by the seller and the deposit (circle option) will / will not be refunded to me.
Trailering of said horse to the purchaser is the (circle option) seller's / purchaser's responsibility.
SELLER:
I certify that this horse has no injury, illness, lameness or disease at time of purchase unless otherwise noted above and am satisfied with the payment & transportation arrangements agreed upon by myself and the buyer.
____________________________________
Seller's Signature
BUYER:
I certify that I am satisified with the health and suitibility of the horse I am purchasing. I (circle option) have / have not had a vet exam done on this horse.
____________________________________
Purchaser's Signature
This contract is legal and binding and should the buyer misrepresent the horse or the purchaser fail to meet the conditions of sale, action in court may be taken against either party.
In the case that the horse has been misrepresented the horse will be returned to the seller and full monies toward purchase price will be refunded.
Dated the _________, day of ____________, in the year _______________, in the city/town of __________________, in the state/province of:____________________, in the country of: _________________________________
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