I've done it several times (as a seller) without a problem. Like several others above, I require full coverage insurance and a non-refundable deposit. These were all "made" (or at least going well) horses, and I knew the trainers the buyers were working with. I wouldn't consider it for a young, green horse.
"Everyone will start to cheer, when you put on your sailin shoes"-Lowell George
As buyers, we have done it several times. We paid for mortality and major medical in case something went wrong. At the end of the trial periods we alway knew (correctly) that we had found the right horse. In each case, the sellers wanted to be sure we were a good match for the horse. We also wanted to be sure.
One other option we have used has been to pay the seller to let my daughter try the horse in multiple situations instead of taking him on trial. I paid one seller to trailer the horse to multiple places, including an event, over a couple of months. He was not an easy horse but was wonderful for my daughter. He is now 27, retired, and still well loved.
I won't do them. I'll make the horse as accessible to the buyer as humanly possible including hauling it to different venues for them, but they don't leave my care until they are paid for.
I have a friend who recently gave a buyer a month long trial - green rider, made horse. At the end of the trial, horse was purchased. Two months later, green rider wants to return it after falling off. Seriously?!?!
If you believe everything you read, better not read. -- Japanese Proverb
The last two I sold went out on trials, and I think it was a good way to get them sold after months of little interest.
Both went out with a similar signed contract; the first was paid in full and would be refunded if returned in the condition he left... the second (more expensive) paid 50% up front that would be kept by me in lieu of insurance if anything happened to him. (They said the insurance company wouldn't sell them insurance because they weren't the owner... I'd never heard of that before.)