Well this sounds awfully familiar...last June Dh and I were looking to purchase a trail horse and we looked everywhere, we finally came across one and it was like we hit the jackpot, it was hard not to jump the gun and go straight for it, we went to see the horse and agreed on putting the deposit down (500 bucks) which btw I got a receipt for. I made the appointment with the vet for the checkup and all the while the owners were saying I really didn't need the vet check cause she had just had one and they had the paperwork, but a friend insisted that I should get my own done no matter what. I got a lot more emails from the owners who told me this horse was registered and had jumped before and had been a good winner and so we exchanged a ton of emails back and forth. I had to wait two weeks before I could get the vet to go down to see the horse and so dh and I went to visit the horse 2 more times while waiting, on the second visit, they asked us if we happened to have room for a companion mare. They sweetened the deal by a whole lot, they offered up the horse for 1500 dollars if I took them both the following week, so I really would only need to put down another 1000 and then trailering which funny enough they offered to do at no charge,  a whole 2 days before the vet was available to check them out. I told them I would think about it and on the drive home Dh and I were discussing this and we were saying how the horse looked like she was in great shape and stuff. 
then the shoe dropped, the day the vet was to be there, I got an email from them, "we just sold the horse and the companion mare, sorry for the inconvenience, we will mail your deposit back"
I was furious.......I drove down there 3 times in 2 weeks, its a 6 hour round trip each time and I was set on the horse....
I did email them back but never received another email from them. I received my deposit back a week later. 
in September my friend was looking into adding another horse, and she called me. I got an email from a  rescue center and I think you should come see it yourself. I drove to her house and the email contained photos of the horse I was to purchase and the companion mare and basic info, horses found at auction bound for slaughter house, rescued, and need info, if you recognize these horses please contact....
I went to the website and they have a full disclosure of health issues and future prospect....it stated that due to the lengthy list of health issues  neither horse would ever be suitable for anything other than companionship. the horse I was to purchase needed boots and had something wrong with its back. 
Sad thing to me was that she didn't look unhealthy at all when I had seen her, so I am not sure what occurred to them in the 4 months since I had seen them last. Or whether it could have been present before and I just didn't notice it, which makes me feel even worse. 
either way I contacted the rescue center and I gave them the info and the location of the owners, I forwarded some emails pertaining to them and  I didn't hear anything at all until late November when the center emailed me a follow up and said they had been adopted to a good retirement home, and that the owners had sold the horses to the first person that came with the 1500 dollars they were looking for, but claimed the knew nothing of the horses after the sale or how they ended up at auction. 
I still can't really say if I saved myself a ton of money and heartache, or if just kind of allowed this to happen by being persistent with the vet check up.