One trainer sending a horse on trial with another trainer is more common in the hj industry, less so private seller to private buyer.
The only time in recent years I let a horse go on trial, the horse was going to a buyer who boarded and trained with a relatively locally well-known trainer at a nice facility. I had a certified check in hand, I had proof that the horse was insured for the duration of the trial, and they had a vetting already scheduled. They were also within 30 minutes. Horse “sold” before the trial was over, as soon as he was vetted. It worked this time, but I would still be hesitant to do a trial except under very specific circumstances.