The verses discuss selling according to the years until Yovel, i.e. a sale of land. Ona'ah does not apply to land!
Ramban #1: Pesukim 14 - 16 are speaking (not about Ona'ah, 1 but) about the law of Yovel
Why does the Torah reckon the years "after the Yovel" and not from the Yovel itself?
Seforno: Because it is unwise to purchase a field in the Yovel itself, when the seller may return and retrieve it before the purchaser has derived any benefit from it .
What are the connotations of "be'Mispar Sh'nei Tevu'os Yimkor lach"?
Rashi: It means that the seller sells, not the field per se, but the number of years of harvests (i.e. excluding Shemitah years). 1
Rashi (from Erchin, 29b): The Pasuk is teaching us that the seller is not permitted to redeem his field from the purchaser before two years - even if the field yields three years' produce during that time.
Ohr ha'Chayim: This teaches four laws; the Rambam rules like all of them. (a) The sale must be for at least two years of harvests; year of drought, withering 2 or Shemitah do not count. (b) The Torah mentions buying and selling to warn both the buyer and seller. 3 (c) The buyer must keep it for two years, even if he gets three harvests within this time. (d) If he gets only one harvest before Yovel, he does not return it until another year's harvest after Yovel.
Rashi: And that consequently, if the seller charges more than its value for the few years until Yovel, or the purchaser pays less than its value for the many years until Yovel, he has transgressed Ona'ah.
Seforno: As is the law of someone who rents a field.
Bearing in mind the fact that Karka is not subject to Ona'ah, as the Gemara in Bava Metzi'a, 56a, learns from Pasuk 14, how will we explain the current Pesukim?