How did Avraham acquire the Me'aras ha'Machepilah?
Rashi (from a Kesav Yad): He made a 'Kinyan Chazakah of the borders'.
Rashbam: By means of a Kinyan Kesef. 1 Ha'Emek Davar (17)
Rashbam: We learn Kinyan Kesef from 'v'Nasan ha'Kesef v'Kam Lo'. (The Gemara cites this in 10 places, but it is not a Pasuk, It is based on "'v'Yasaf Chamishis Kesef
Why does it say "l'Einei Bnei Ches"?
Malbim (17): All were there, and no one protested, e.g. to say that he is a neighbor and has first rights to buy.
Ha'Emek Davar: They should not be able to say that it is a disgrace to sell an inheritance, and it is invalid. It was [in front of] "b'Chol Ba'ei Sha'ar Iro", so Bnei Ches could not deny [and say that they were unawars of the sale].