I think a trial period could work..but not in every case. I was selling a horse, recently, that is at a friends house. She was screening people and a woman called saying all the right things: experience with the breed, a love of horses, experienced confident rider, had the money to spend, blah, blah, blad.
She pulls in with a fancy rig after driving hours, with her own horse and is going to ride alongside my horse for sale, to see how she acts. If all is good, they'd switch horses. Well, all they did was walk, no gaiting, she was too scared, she had some disabilities and couldn't mount from a block, etc so she wanted to bring the horse home so her trainer could evaluate them together. She said she'd insure the horse while she had her and if it didn't work out, she'd bring her back.
After some conversation with my friend, I found out that she never once even touched my horse. Not a pat on the head, not a hand on her. She never even really addressed her, just referred to her. There's NO WAY I'd allow that woman to take that horse, even if she paid for her in full, tried her and if it didn't work, bring her back. If things were different, I might have reacted differently, but, no way was the horse leaving.
I've been offered a two week trial with a horse and for me, on the buying end, I felt it really honest of the seller. To see if this horse and I got along ok and if we didn't she honestly had another buyer in mind. Because of that, I trusted what she had to say about the horse. Again, it could work, just depends on the situation.