In response the the question about how as a seller, you match horse and rider...
I always ask potential buyers what they are looking for, what they want to do with a horse, and what they've done in the past. I do this via e-mail or phone, and based on their answers I either tell them to shop elsewhere or I describe one or more of my horses.
I slant my description of the horse to test them about issues that are up in the air, to see how they react. I've found no absolute rules. Sometimes people actually like challenging horses even if it's not going to get them where they want to be competitively very quickly. I've had a number of riders who got on horses that did everything they asked, and impressed the heck out of the mom and the trainer and then surprised us all by saying, "I just don't really like him that much."
It can be frustrating to everyone else, especially if they've flown halfway across the country to look at this one horse, but I've come to respect people's gut feelings.
I've also had people fall in love with horses that I first thought were too much. I make sure they understand all the horse's weaknesses and training issues, but if they want the challenge, especially if they're adult and can afford training, then I respect their instincts. I must admit that I've had horses that showed me a lot of resistance who adapted to the less decisive riding styles of their new owners and did wonderfully.
Some trainers, especially in the hunter and equitation worlds, insist on making buying decisions for their clients. In eventing, this is fortunately less common. Most people need and should have a trainer or some expert third party for advice, but I firmly believe that there needs to be room for the horse and the rider to make the final decision about the other (the horse does it as soon as the buyer walks into his stall).
You know that quote about the best kept secret is that between a horse and its rider? It's true, and sometimes those of us on the outside just don't see it right away.