My two cents: I would be extremely wary about buying from an individual track trainer I don’t personally know well. As @Jealoushe noted, their fiduciary duty is to the owners; thus their priority is to get these horses off the owner’s payroll. Plus, you don’t have to actually know much about horses to pass the exam for a trainer’s license in my state, at least.
I work with a retired jockey. He still has connections at tracks all over the east coast. Because he’s retired, he isn’t beholden to any of them for his paycheck. He occasionally picks up a cheap ($500-1000) project when he’s bored. None of those horses were ever advertised – all word of mouth & all people approaching him with horses & not vice versa. (Seriously, the dude’s phone pings at least twice a day.) They’ve all been safe, safe, safe. He’s too old & sore to want to get banged up riding a loon anymore. His wife & kids are good “civilian” riders. So, while he hates riding in an arena & may not get the nuances of normal riding, he’s still very realistic about what constitutes a dangerous horse off the track. I trust him enough that if I liked what I saw in video/pics of a horse he showed me, I’d feel ok buying it sight unseen.
The riders usually know which horses are trouble, which horses broke down, etc. How freely they can talk about things if they’re currently riding, I don’t know. And not everyone lives in an area like this where you’re likely to know somebody currently riding racehorses or retired from it. Worth keeping in mind, though.