But that's not true. A business seller does not "require", legally, any due diligence from the buyer. Legally, the horse has to be what a business seller says it is.
It's really wise to do your due diligence whoever the seller is, but the law is clear. A private seller can legally unknowingly sell a horse which is not fit for purpose but a business seller cannot.
I'm sorry FF, I'm not trying to be contrary about this, it's a very important point of law. It's what makes horses bought from a dealer (not as an agent, owned by the dealer) worth more than a horse from a private seller.
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Again, details. The horse might be what the seller said it was. IF she said it measured 2 years ago or at age x or whatever it was. With one side of the story, and no hard evidence (ad, communications, etc.) We don't exactly know.
I don't know that the seller ever said "pony currently measures under/within to qualify as a pony."
If I were selling I would've said, she measured at x time, but now I'm not so sure. If you are looking for a pony to compete, a measurement should be apart of your pre purchase exam. Or I as the seller would've done that to know what I'm selling. But I'm honest and it often makes me lose money 😆
If the seller writes "For sale: pony that measured under x height" that's not technically wrong. The pony did measure under...at one point. The seller did disclose this (that the measurement was older). Is it a bit misleading? Possibly. Enough to win a case? Probably not, but depends on a few things.
My point is, we can play armchair lawyers all day and we all want the best for the OP as well as the horse, but we just can't say for certain. That's all I'm saying.
If you hold the strong opinion that the buyer can take it to court and win, that's fine. It's your opinion. There are other opinions out there that also present plausible outcomes though.
The thing with the law is that "it depends" and can seem black and white, and sometimes it is, but often it is not.
While I don't practice UK law, exactly (Int'l, anti-terrorism, European), I do have experience with raising my eyebrows when reading decisions of the court 😅 and know the devil is in the details.
For the OP, it is definitely worth talking to a solicitor. Then depending on that outcome, bringing it to the court.