Research the new owners!:
You know your horse, its needs, temperament and abilities but do the prospective new owners? Make sure you question them extensively on their equestrian knowledge, finances (horse ownership is very expensive) and their experience of horses and horse ownership.
Healthy horses and happy hackers!:
Horses all need different types of specialist care as well as suitable places to live and exercise. Please make sure any prospective new owner knows all of these and has access to specialist veterinary care should it be required. New owners should also arrive with a vet and suitable transport as the horse must be checked before it leaves you.
Make sure the new owner is impulse buying as the horse may be neglected once their interest wanes or they realise the costs involved in horse ownership.
You should ensure that the prospective new owners are aware of the long term time and financial commitments of horse ownership, are they ready for it, have they researched it and do they have facilities in place for the accomodation and exercise of the horse?
Don't be afraid to say no!:
If at any time, you don't feel comfortable with the prospective new owners, then just say no, your horses health and wellbeing are more important than anything else, its upto you to see these are maintained.