When I bought Sonny we didn't really do any research on the stables...my trainer new the owner of the stables, and a co-worker knew him also and though only my co-worker recommended him, I still got a good horse
When I was horse hunting, I've walked away from 2 horses that I really wanted to buy because they said that to ride the horse again I had to do a lesson or pay them for their time....which IMHO is stupid, if you are selling a horse you should want the prospective buyer to be comfortable with that horse, even if that means the person riding the horse 10 times before making a desicion.
At first, before we actually bought Sonny, we were about to walk away because of the agent not wanting us to speak to the owner at all...and the agent didn't have a signed piece of paper from the owner that she was allowing the horse to be sold through the lady.
Thankfully, we went to a local tack shop and the owner asked if I had found a horse yet (she knew I'd been looking for one) and I told her about Sonny, but about no papers saying the lady had permission to sell the horse...but the lady (whom I do trust) said that the owner (whom was a really frequent costumer there) was selling her horse and I was able to get her name and number if something happened where I could contact her.
Of course if I was ever put in that position again, I would walk away from it, but it was my mom's idea to still buy Sonny. Luckily the lady did want to sell Sonny and we've had no problems since.