How long have they had the horse
Why are they selling
What does price include - tack, rugs etc. If so, when was saddle last fitted
What bit is the horse ridden in & why
What's horse like with farrier, dentist, to load, tack up etc
Is horse shod? Any known probs with feet
Medical history
History of sweet itch
Are jabs up to date
Are teeth up to date
Is horse chipped/freeze marked
Does horse ride okay in traffic
Does horse ride okay in open spaces
What does horse hate/spook/nap at (eg dogs, tractors etc)
History of bucking, rearing or bronching
Does horse prefer schooling or hacking
Live in and/or out
What is horse fed, how much & why
History of weaving, cribbing or windsucking
Depending on age, has the mare foaled previously or when was he gelded
I think that would be about it to start with!