I've only bought, never sold, so that's the perspective my opinion is from
When I bought my horse, every trainer at my barn told me NEVER to buy a horse without a trial, and I'm glad I took their advice.
I took one horse home on trial who turned out to be INSANE once she got away from her owner- was too dangerous to complete a vet check, stall cleaners wouldn't go in her stall, she wouldn't pick up any of her feet, etc. She was going to need to be sent to a trainer before she would be safe for me to handle. She went back to the seller instead, who refused to give me my whole deposit back (even though the contract specifically said it was 100% refundable), but it wasn't worth taking to court...
The horse I ended up buying I got from a trainer/breeder recommended to my trainer by a show judge who has used him for years. Instead of a trial, I bought the horse outright with a 30-day return policy (and this one was upfront about keeping some of the purchase price for delivery/pickup if it didn't work out). I wouldn't have accepted the terms of all money up front if he didn't have a long-term business relationship with someone my trainer knew well. The horse was definitely a keeper, though
If I were to sell a horse, I'd definitely require a deposit for a trial period, but I'd probably push for the return policy instead if the buyer wasn't very local. I'd also want the buyer to visit the horse multiple times before taking him anywhere.