As a teenager I worked for a dealer for just over 12 months.
I found that the horses that came through that were genuine - sound, fairly quiet and nice enough looking, went for crazy amounts of money. The usuals off the back of the doggers truck and off the track that were nut cases or unsound, we sold for over $2000 - still a lot of money for a dodgey horse!
I was told to get on these horses as a bit of a crash test dummy, do some work with them and within a week they'd be advertised as quiet, young riders horses with dressage potential. I'd be instructed to forget about my correct training, and just get their heads in and down asap, and have it go walk, trot and canter and a bit of leg yield or any other 'trick' that I could force it into before being advertised in a week or two.
I was left to deal with people coming to look at the horses. My boss would drug them before clients arrived, instruct me to sell it or I wouldn't get paid, and then just take off and leave until the people were gone. If any uneveness showed up through the drugs, I was to say that the horse was just foot sore as we'd just taken it's shoes off.
Before working there, I was seeing the horse world very much through rose coloured glasses, so this certainly woke me up to how things are done in that industry.
Seeing how it all works, I would not go through a dealer to buy. As someone above has said, they do not know the horse's full history and if they do, they often will not disclose it. After all, it's their job, their income, their livelihood, if they don't sell horses as quickly as possible, they don't make money.