If it's important(as in, I would ever want to 'breed show'or be able to 'measure in' as a VSE), I would no longer accept any other person's measurement as a basis for purchase, UNLESS I was standing right there and could verify it as done.
Accurate measurement begins w/ a PROVABLY level surface; I personally believe that many do NOT have access to such, so you can be defeated before you begin. IF and WHEN you have that, there is also: an accurate/official measuring stick, knowing and DOING what constitutes 'standing up square', with head in a normal, natural position, keeping hands off horse and pressure off lead. For all this you must KNOW the measurement rules, scrupulously follow them...AND, have a horse who has been taught to have MANNERS in hand(which is job one of the handler/trainer/owner; IMO, it is NEVER an 'excuse' that the horse 'wouldn't stand still'...). Finally, the horse should have a PROPERLY trimmed foot for an ACCURATE measurement...I mean, NOT overlong toes and/or heels, but NEITHER a horse trimmed too SHORT(thus requiring masking painkilling drugs, which are both illegal AND UNETHICAL), and which is cruel to the horse. You may have the questionable 'luxury' of leaving extra length ONLY when the horse is comfortably UNDER the maximum allowed height; even then, what about the effect on the horse's tendons/ligaments...soundness, both present and future,by such practices??
All this said, I am always bemused by the 'possible justifications' that are presented by various posters EVERY time this subject is brought up here(and I have seen it come up numerous times in my over-13 years as a member.)I have NEVER personally experienced having a horse 'measure tall' because it was tense, cold, hadn't been 'worked' first, etc., etc., etc. Not saying it is NEVER possible, but I believe, not nearly as likely as often suggested-and why would you not give ANY horse a chance to be 'naturally' more relaxed,and/or warmed up a bit, before you'd attempt to measure it, IF you believe that might make a difference?
I HAVE personally experienced AND observed that it is 'possible'for moving the horse around and remeasuring to make a 1/4" difference, and in VERY rare cases, I have observed a nearly 1/2" variation...but nothing like the difference as has been found here. I am convinced that there really is NO WAY that any of the 'possible explanations' mentioned within this thread would make a full TWO INCH difference; on most 'actually' 34" horses, that would be more than roughly 2/3 of the entire 'proper' front length(coronet to edge of toe)on an 'average' and 'normal' length hoof!
To the OP: I am very sorry this happened to you; afraid it is sadly true nowadays that you need to get EVERYTHING in writing, as suggested by some. You also may find it necessary to take legal action in civil court, if that's in any way feasible.If you think it possible to work out w/ the seller, continue to politely try? If this doesn't help... as sfmini suggested, perhaps share the name and description/registration number of the horse you bought recently, w/o additional comment? I know this is a tough position in which you find yourself.
Best wishes,
Margo