If you can possibly travel to try the horse, do. A short trip to Europe costs far less than one wrong horse shipped across an ocean, and there is no real substitute for feeling an animal underneath you. When you go, follow a proper order rather than just hopping on.
See the horse in its stable and handled on the ground first. Run your hands down its legs, watch it walked and trotted on a hard surface, notice how it behaves being tacked up. Then have the seller’s own rider ride it before you do. This is not optional, and a good seller expects it. It keeps you safe on an unfamiliar horse, and it lets you watch the horse’s honest first minutes and see what it can look like in capable hands before you climb aboard.
Then ride it yourself, to a plan. Walk on a long rein first, then work through all three gaits and plenty of transitions on both reins, and ask for the movements the horse is meant to know. Somewhere in there, make a deliberate small mistake and see how the horse takes it. For any horse you are serious about, come back and ride it a second time on another day, because the first ride is a first impression and the second one is information. The test to trust, in the end, is simple: do you want to get back on tomorrow? If the honest answer is a polite “it was fine,” keep looking.
Not everyone can fly to Europe on short notice, and for buyers in the Gulf, China, India or the far side of the United States, a trial trip is a serious undertaking. This is where a proxy rider earns their keep. An independent professional who understands your riding can go, handle and ride the horse, and give you the feel and the temperament read that video cannot, so that you are buying on real information rather than hope. It is one of the services we offer for exactly this reason, and even if you arrange it elsewhere, insist on knowing, in writing, who the rider answers to and who is paying them. An opinion whose loyalties you can’t verify is just more advertising.