There is a misconception here that appears to be shared quite commonly. Adam and Eve's sin was in disobeying God's command not to eat from the tree of the knowledge of good and evil.
Since the tree contained the knowledge of both good and evil They were clueless as to what was evil, but were also clueless as to what was good. So actually it may be said that at that point is when man gained the trait of self will.
Up until they ate from the tree there would have been no consequence for disobedience since only eating from the tree was forbidden and had a God given them the consequence. I am not saying that they disobeyed God before that only that God had not ascribed any other punishment to the best of my knowledge.
Sin is, in it's most basic form, disobedience of God. Although Adam and Eve did not have the knowledge of good and evil, they did know right from wrong, as is evidenced in:
Genesis 3:2 and 3 And the woman said unto the serpent, We may eat of the fruit of the trees of the garden: But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall not eat of it, neither shall ye touch it, lest ye die.
So from this we learn that Eve and by extension Adam knew that it was right for them to eat of the fruit of the trees of the garden, but to eat of the tree of the knowledge of good and evil was wrong.
The following Scripture shows that Adam was well aware that eating from the tree was wrong, even before he had gained the knowledge of good and evil:
Genesis 3:17 And unto Adam he said, Because thou hast hearkened unto the voice of thy wife, and hast eaten of the tree, of which I commanded thee, saying, Thou shalt not eat of it: cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life;
So in effect, Adam had obeyed his wife instead of obeying God.
their punishment was for disobeying his command not to eat of the tree which they both knew they were not supposed to do. In disobeying God man gained the trait of self determination, or in other words at their decision to disobey God they chose death, conversely when we chose salvation we chose life.