If it is OBVIOUSLY a material consideration of the seller's that the horse get a good home, such that you know that the seller would not sell to you if you told them your true intentions, it is not acting in good faith to take a horse under promises of "trail riding home" and then sell to auction less than two weeks later. The promises of "trail riding home" are PART of the consideration/sale price for the horse. You can induce a buyer to lower their price if the promise of "trail riding home" is more important to them than the money they could get selling to a lesson barn down the road. Legally if they lowered their cash price to you because of the promise you made, you owe them the difference. Just because it is difficult to enforce doesn't mean you don't bear legal responsibility for making someone change their bargaining position based on your lie.
Just because it is difficult to ENFORCE legally without written documentation, does not mean it is perfectly within everyone's rights to outright lie to seller to get them to sell you a horse. What is hard about this?!
And aside from the fact that when promises become part of the "consideration" in a horse deal, there is both moral and legal (even if difficult to enforce) weight to them, I get a little tired of everyone bending over backwards to justify and explain away the absolute lowest common denominator of behavior.
Jesus. It is WRONG to lie to people in order to get them to sell you a horse they would not otherwise sell you if they knew the truth.