Hmm. What a good question.
I was able to get a 2 week trail period on my Mustang. He was $2000. Best $2000 I ever spent- I knew I wanted him after a week.
The guy was a friend-of-a-friend. I don't know if that made a difference in his decision to let me try the horse out or not.
My last horse I took a gamble on. The guy let me test ride her, but that was it. It was a take-it-or-leave-it type situation. The horse was going to auction if I didn't buy her, so I don't think the guy really cared if I bought her or not. She was only $500. I ended up buying her because my friend promised me that she would buy the horse from me if it didn't work out. She had a lot of faith in her horse judging skills I guess. Me, I wasn't so sure, but didn't really have much to loose, so I did it and bought the horse without a trial. Definitely made me nervous though! But the horse worked out fine.
If I were on the other end, I don't know. I really don't feel comfortable with a trial, but on the other hand, if I were to "sell" a horse it would probably be a situation where I would give the horse away to a good home. If that was the case, I would make sure the people knew that if the horse wasn't working out that I would gladly take it back.
So I can definitely understand why a seller wouldn't want to risk it when they could probably sell the horse without a trial to someone else. But as a buyer a trial period is great.
I think the best case senario is to buy a horse from someone you know who will let you try the horse out first. Then you are (hopefully) both comfortable with the situation. But not everyone can buy a horse they want from someone they know so that only works occasionally.
But I don't think a seller refusing to do a trial period necessarily makes the horse a lemon. It could just be they don't want to take the risk.