Observe the owner with the horse. Are they frightened of the horse? Are they negligent while handling him? Ask the owner about soundness and behavioral problems - yes, they can lie, but at least you asked. (If, later on, the horse needs to be returned for whatever reason, the seller can't say, "Well, you never asked.")
Always try to bring a knowledgeable horseperson with you so that they can watch you ride the horse and offer their opinion. If this is not possible, ask to see the horse being ridden before you yourself climb aboard. Gait and other such problems are often hard to detect when you are riding a new horse, but are usually easy to see. Before you climb aboard, always wear a helmet. Accidents can happen even with horses you know well, but with a strange one you should be doubly careful. Unless you are like the character Steve Martin played in The Man With Two Brains and have one to spare, don't risk it!
If at all possible, take the horse out on a riding trail; many horses behave quite well in an arena, but panic at the sight of swaying weeds alongside the trail.
You might be surprised at how many knowledgeable people overlook the importance of even a simple cursory examination when purchasing a horse. A friend of mine who has several years experience with horses recently bought a gelding from out of state. She was told by the seller that the horse had "a little hitch in his giddy-up," and assumed it meant the horse was a bit stiff at the canter - not unusual for an older horse. Trusting the person who recommended the horse to her, she didn't ask for specifics and she didn't request a vet-check. It turns out the horse had irreparable spinal damage and was completely unrideable. The problem was instantly noticeable, and had my friend even requested a video of the horse moving in all three gaits and backing up, she would have seen it. A vet observing the horse would have advised her against purchasing it and she would have saved herself a lot of trouble and heartache.
Once you have agreed to buy a horse, make certain that you have a contract outlining the terms of the
