This is a very good post. It is extremely common for horses to “change” post-purchase. Horses are sensitive animals that often behave and function quite differently in different environments and different programs and under different riders. I would say that the vast majority of the time, no drugging took place. Most likely the seller showed the horse to the prospective buyer at its best–in familiar surroundings, handled by familiar people, and tuned up by a rider experienced in schooling it to it’s best advantage. Then after the sale, the horse is separated from its equine friends, from its comfortably familiar surroundings, handlers, and routines. It can take a horse a long time to adapt to new routines and understand what the expectations are in its new home. Also, a submissive horse that is separated from its old herd mates and moved to a new home can quickly take on a new “dominant” persona as a natural response to a move to a new home.
People are quick to suspect that a horse was drugged, and then after that people suspect the change in feed had an effect. Personally I think that those two things are rare culprits. I think that the bigger issues tend to be the response to new riders/handlers, changes in work/training routine, and other environmental factors such as having suitable turnout companions. The confidence level and riding style of the rider can be a huge change to expect a horse to adapt to. Horses also need to find (and become comfortable with) their new social standing in their new home.
ETA: I’m discussing this because while I think that it is totally reasonable for a buyer to be wary of horses that may have been medicated, I also think that when you have a client that has had a horse change drastically after purchase, it’s really important to examine and reflect on other potential root causes, and also consider purchasing a slightly more seasoned animal that has already been successful in more than one environment/barn.