"For what I seal on earth, shall be brought against you at the judgment bar." What is Nephi talking about? He is putting his personal seal on the truthfulness of his words and of those contained in his record. These words will then come forth and be part of what we all will be judged by. Knowing the words of this record, we are now accountable.
"For thus hath the Lord commanded me, and I must obey. Amen." That is just typical, reliable Nephi. "I will go and do the things which the Lord has commanded." From the very beginning to the very end, obedience has to be one of Nephi’s great qualities.
H. Dean Garrett, "Nephi’s Farewell," in The Book of Mormon: Second Nephi, The Doctrinal Structure, ed. Monte S. Nyman and Charles D. Tate, Jr. (Provo, UT: Religious Studies Center, Brigham Young University, 1989), 377–390.