Why does the Torah add the word "la'Amisecha" both in connection with selling and in connection with purchasing?
Rashi #1: This is the way the Torah speaks.
Rashi #2: To teach us that wherever possible, one should purchase from one's fellow-Jew.
Why does the Torah insert the word "mi'Yad"?
Ramban #1 (citing Bava Metzi'a, 56a): to confine the Isur Ona'ah to Metalt'lin. 1
Ramban #2: This teaches us to draw a distinction between Metalt'lin and Karka - with regard to the Din of Ona'ah. 2
Why does the Torah write twice the La'av of Ona'ah (here and in Pasuk 17)?
Rashi, Ramban and Seforno and Targum Yonasan (on Pasuk 17): Ona'ah in this Pasuk refers to Ona'as Mamon (cheating and overcharging), whereas in Pasuk 27 it refers to Ona'as Devarim. 1
What are the ramifications of "Lo Sonu"?
Ramban #2: It is a warning that the two parties should clarify to one another how many years there are until the Yovel, and to stipulate that, in the Yovel, the field must go back to the owner.
Seforno: "ve'Chi Simk'ru Mimkar ... " refers to the seller cheating - such as by removing the chaff from the top of the pile but not from the bottom; 1 whereas "O Kanoh ... "refers to the purchaser taking advantage of the seller's ignorance of the price, and offering to pay less than the market price. 2
What is considered Ona'ah? What happens if somebody transgresses?
Ramban: If it is an amount less than a sixth, the sale remains intact and the transgressor is not obligated to compensate the other party. 1 Whereas if it is a sixth, the sale is valid, but the transgressor is obligated to return the balance.
Ramban: Because for such a small amount, he is Mochel. Nevertheless, the transgressor has contravened the La'av of Lo Sonu. In the event that the amount involved is more than a sixth, the sale is rendered invalid.
What is the reason for the sequence of all the current Parshiyos?
Rashi (later in the Parshah, Vayikra 26:1): First the Torah warns about Shemitah (and Yovel); in the event that someone has a lust for money and is suspect on Shemitah, he will be forced to sell his Metalt'lin (movable goods [Pasuk 14), then, his fields (Pasuk 15), then his house (Pasuk 29); If he continues to transgress, he will be forced to borrow on interest (Pasuk 35); and finally, to sell himself, first to a Yisrael (Pasuk 39), and then, to a Nochri (Pasuk 47). 1
Rashi: Each punishment is worse than the one that precedes it.