if you can avoid it, do it. Buy from another perspective, if the horse is easily mentally damaged or scarred and this is one of his first experiences trailering and you have no options to train him to trailer in the mean time, it could be a major help. think about it, forcing a spooky horse to trailer the first time with no prep, the horses gets hurt on the trailer having a freak out, you now have the rest of the horses life to deal with being a 5 hour excursion to get him on because he`s traumatized. or you could give him ace, a small amount, keep it pleasant, and work on it later. all depends on the horse.
If something is not to your liking, change your liking- the only thing you can count on in this world is change. ♥