I highly suggest against it, but if there is no way you can see a horse you've fallen in love with, I highly recommend doing the following:
- Ask to see a few recent videos (very recent, not "oh, this was shot a year ago") of him being worked with; i.e. groomed, led, lunged, turned loose (to see gaits), and ridden in whatever they advertise him being trained in. Anything that is important to you (ground manners, movement, temperament, willingness under saddle etc etc) ask for a video of.
- Get a vet check from a reputable vet that does not work with that farm. You want an unbiased opinion on the horse.
- Get everything you agree on in writing and signed by both parties - express mail is your friend in this case. For example, if the horse is guaranteed sound for 30 days (barring injury), or guaranteed in any way, get it in writing. More than that, ask the party to initial each and every point, so they can't claim that you changed a portion of the contract.
Red flags to watch out for:
- Getting the brush off; if the seller won't answer questions or seems rushed to get you off the phone, walk away. Same goes with answering certain questions, or refusing to get current photos or videos. If you show that you are absolutely serious about purchasing this horse, they should be doing everything possible to make you 100% comfortable with the purchase.
I'll add more as I think of it :)