Having repeated the details of Me'aras ha'Machpeilah, this Pasuk appears entirely superfluous!
Oznayim la'Torah: The Torah is hinting here at the Midrash, that all those who participated in the burial of Avraham, merited to participate in the burial of Sarah, although it took place thirty-eight years later! 1
See Oznayim la'Torah for details.
Why does the Torah repeat that he bought it from Bnei Ches?
Ha'Emek Davar: Had they not agreed that Avraham make it a cemetery, they could have protested (that Avraham bought only rights to one grave). All reasons do not justify any illegality about a burial site.