Answer: Melchizedek was a believing Jebusite during the days of Abraham, and he happened to be both king and priest of Jerusalem. Because he was both king and priest, he became a type of the Messiah. However, he was not the preincarnate Messiah since theophanies merely appeared and gave their message and then disappeared and did not hold any type of permanent office like being priest and king. Furthermore, Hebrews 7:3 specifies that he was but made like unto the Son of God (emphasis added), meaning he was not the Son of God.
Furthermore, Hebrews 5:1 clearly specifies that only a true human being could become a priest. The second person of the Godhead did not become human until the incarnation in the womb of Mary.
So, Melchizedek was a king and priest and a type of the Messiah, but not the Messiah Himself.