Mishnah 8 opens with a general rule: "Kol makom sheyesh mecher - ein kenas" - wherever there is the possibility of sale, there is no fine. That is, in any situation where the father has the option of selling his daughter, there is no obligation of a fine. To understand this, we must first introduce the concept of an amah ivriyah, a Hebrew maidservant.
Selling a daughter as an amah ivriyah:
The law of the amah ivriyah appears in Parashas Mishpatim: a father may sell his minor daughter, and this sale carries within it the possibility of developing into marriage, a sort of prelude to marriage. He has this right only while she is still a minor, below the age of mitzvos, for once she reaches the age of mitzvos she goes free. This is therefore a law that applies only to a young girl.
From here the Mishnah establishes: as long as there is the possibility of sale, meaning while she is still a minor, there is no fine. A girl who was raped or seduced while still a minor, below the age of twelve, carries no obligation of a fine. There may be other obligations of damages, but the fine of fifty shekels does not apply.
The view of Rabbi Meir versus the view of the Sages:
The Gemara explains that our Mishnah was taught according to Rabbi Meir, who holds that there is no fine for a minor. The Sages disagree and hold that there is a fine even for a minor: from the age of three and onward, from the point at which relations with her are legally significant and have the status of an act of relations - whereas before that they have no such status - both the fine and the sale apply to her, until she reaches the age of twelve.
The Mishnah continues: "Vechol makom sheyesh kenas - ein mecher" - and wherever there is a fine, there is no sale. Once she has reached the age of twelve and become a na'arah, she can no longer be sold, since the law of selling a daughter as an amah ivriyah applies only to a minor. On this point all agree.
The three stages the Mishnah details:
"Ketanah yesh lah mecher ve'ein lah kenas" - a minor is subject to sale but not to a fine: she can be sold, and the father does not receive a fine. And as stated, this is the view of Rabbi Meir.
"Na'arah yesh lah kenas ve'ein lah mecher" - a na'arah is subject to a fine but not to sale: once she has reached the age of twelve, during the six months until she becomes a bogeres, she is subject to a fine and can no longer be sold. On this all agree, except that the Sages hold that even below this stage, before she became a na'arah, she is subject to a fine.
"Bogeres ein lah lo mecher velo kenas" - a bogeres is subject neither to sale nor to a fine: once she has reached the stage of adulthood, six months after becoming a na'arah, she cannot be sold, since the law of sale already lapsed when she became a na'arah; and she has no fine either, for the fine applies only when she is both a na'arah and a virgin.