Why is an animal disqualified once it is sanctified?
Reb Chaim, I assume you are referring to what Rebbi Yochanan said in Kerisus 27a, that if one of the partial owners of an animal sanctified his half of the animal, then bought the other half from his partner and sanctified the second half, the animal is sanctified but is disqualified from being brought as a Korban. This is because Rebbi Yochanan's opinion is "Ba'alei Chayim Nidachim" -- if an animal was at one point invalid as a Korban it cannot later become valid, because it has already been "pushed aside." Therefore, since at one stage only half of the animal was sanctified, it clearly was not possible at that stage to offer it as a Korban since half of it was Chulin. Since at that point it was "pushed aside," it follows according to Rebbi Yochanan that it is too late, and it can never subsequently become re-qualified.