-Before you buy a horse, you normally call to see when you should go look at him. ALWAYS show up ahead of the time the seller told you. You want to be there to see them take the horse out of the stall and tack him up. This way you can check for grooming/tacking vices AND you can make sure that the seller IS NOT giving the horse any drugs before he is being ridden (i.e calming, etc). I have seen people who bought perfectly levelheaded horses who turned out to be NUTS after the next day because they were drugged prior to buying.
-Some horses who have soundness issues loosen up and do not seem unsound after being ridden for a while. This is another reason why it is important to not only watch the horse come out of his stall... but to also watch him being WARMED UP and RIDDEN.
-Check to see if the horse is wearing any kind of shoes. If not, can he wear them? If so, what kind are they? If they are some sort of WEDGE shoe, BAR shoe, etc... Check to see the REASON for why they are wearing them. It could be linked to an unsoundness.
-NEVER be afraid to ask questions.
-Check to see if the horse's conformation is suitable for YOUR discipline. If you are'nt that adept to knowing conformation .. that's what the vet is for.
-ALWAYS have a TRUSTED vet do a check on the horse when you take him out to trial him. The vet will check for any lameness, swelling, blemishes, or conformation problems that may effect your horse in the long term.
-Make sure you ask the owner about if the horse has had ANY extra vet visits for laminitis, fractures, illnesses, worms, etc. Certain diseases such as NAVICULAR are PRONE TO COME BACK... and are not worth risking on a horse that you ride every week.
-ALSO, learn about the horses feed and medications/supplements. Figure out the reason for them and learn the last time they were dewormed/inocculated.
-If any serious hoof problems arise... let your VET or FARRIER look at them while the horse is on TRIAL. NOT after you BOUGHT him.
I know this seems like a lot of information... but, I was very surprised at the amount of people that buy horses without trialing them and having them examined by the vet first. Especially if you are using this horse for breeding or upperlevel riding, it is VERY important to make sure his conformation is suitable and his body is healthy.
Good Luck to all you horse buyers!