There are some "red flags" in dealing with sellers. A "closed" barn or a reluctance to permit a "trial ride" are two of them.
As a seller, however, I have to be a bit careful with buyers. Overstatement of riding ability is very
common. If someone wants to try a Marchador I have for sale I usually put them up on one of our very well trained and experienced horses first so I can evaluate their skill. Then we'll move up.
One way for a buyer to protect themselves against shenanigans
is to arrive an hour or so before
an appointment time. Of course they should be very
apologetic (misread watch, misunderstood time, "phat phinger" used to put appointment in Outlook, etc.). This will often give the opportunity to see the horse "in the rough" and be able to see if they can be caught in the field, grooming manners, etc. It's also a good opportunity to watch the horsemanship skills of the seller. Those are important in evaluating the horse, too.
The horse seller/trader is the spiritual ancestor of the used car salesman. Anyone who touts how "moral" or "ethical" or "humane" they are is one to watch very