1) He/She may work the hell out of a green horse before the scheduled time you have set to view so that it's tired and quiet by the time you show up.
Advice: Show up earlier than scheduled. Also, leave to think about it and show up a second unexpected day and ask to see the horse that you are interested in.
2) He/She may "bute" or "banamine" a sore or lame horse to mask symptoms when showing you the horse.
Advice: Pre-purchase Vet exam. You choose the Vet.
3) Tell you the horse is younger than it really is.
Advice: Know what you're buying. Again, pre-purchase vet check.
4) Tell you this is a perfectly "kid-safe" horse. Their kids ride it all the time!
Advice: Find out what level a rider their child is. Use your gut. Pay attention to the horse's cues. You can tell a lot by watching things like "ears" and movements, how it leads, tacks up, etc..
5) He/She may tranquilize the horse before you come to see it.
Advice: Watch for this. A tranquilized horse may be slow to react. Have a droopy lower lip. Dull eyes. Again, show up a second time, unexpected and ask to see the horse.
6) He/She may tell you that there are others interested in the horse and it may not be there.
Advice: Think about that. Why are they so anxious to sell? Pressure to buy is never good.
Bottom line is you want to watch the horse being caught and led in. You want to watch it being tacked up and bridled and pay attention to the way this horse is behaving. You want to come back for a surprise visit. That will debunk any tricks that may be used on a scheduled visit. You want to see vet records and know why the person is selling. And most of all you want a Pre-purchase vet exam, vet of your choice before buying. And use your gut and head, not heart. Be patient, there's a lot of horses out there for sale.
If anyone wants to add something I may have missed, please feel free!