Genesis 4:6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen? 7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.

Adam and Eve fell from glory to an ordinary life because of sin. If you read carefully you'll see that sin here is a person not an action. In order words, Adam came under the control and dominion of sin. Look at this: sin lieth at the door, and unto you shall be his desire, and thou shalt rule over him.

The word "accepted" here mean to be elevated, uplifting, carried away, to bear iniquity, to forgive. In essence, what God was telling Cain is, if you offered acceptable offering, you will be carried or forgiven. The right thing for Cain to do was to go back and offered right kind of offering. 

Hebrews 9:22 And almost all things are by the law purged with blood; and without shedding of blood is no remission.

It is not because Cain brought the fruits of the field, it is because he didnt offered the right kind of sacrifice acceptable to God. When you dont offer right offering, both the man and his offering are rejected.