In any case, the seller should be avoided as untrustworthy.
If buyer 2 set out to grab the movement from under buyer 1 intentionally...s/he should also be avoided as untrustworthy.
If buyer 2 didn't realize what had happened until after the fact, but then offered the movement to buyer 1 at what s/he had in it at maximum, it's a mess; but buyer 2 is in a better light in my eyes. Not a saint, as I question contacting the seller with an offer after a sale on a rare piece... Chances of seller having more than one? Dang slim. So either way you go, buyer 2 has damaged their reputation, just to what extent?
On these rep forums (and the Gen ones too for that matter), we operate on trust. Once it's lost, it cannot be regained and the forums cannot function without trust.
It is a shame to see what greed will do to the ethics and dignity of some... True colors shining through and all that.