A lot of people will pay more (potentially double) for a horse that is registered with one of the major breed registries.
There aren't breed shows for registered Thoroughbreds as far as I'm aware, so that probably doesn't make a difference in your case.
Health and soundness are a huge factor. A horse can be amazing in every aspect, but if he's not sound, most people aren't at all interested. Safe for anyone to ride increases value dramatically, but at the same time, most people want something between 6 and 12 yrs. The same people who want a beginner child safe horse, seem to think that the horse also should be young. If I were looking for a horse that I could feel good about throwing my very novice kid on, I'd look for something older and preferable on its way to crippled, so it couldn't move anywhere very fast
The most valuable horses, are those with highly sought after breeding, good conformation, the training and talent to perform competitively at the top level, but easy and sane enough for a non-pro to win on. Depending on the sport a horse like that is worth at least $30k and on up.
The easiest horses to sell are sound, safe, gentle, registered geldings, between 5-10yrs old, who have enough solid training to be competitive at mid-level competition. Generally those types are within the $8k-$12k range. If we get any like that in to sell here, they're scooped up pretty quick!