Why does the Torah make such a big story about Avraham Avinu buying the Meoros Hamachpela? Rabenu Yona explains that this was the last of the 10 nisyonos of Avraham Avinu. Hashem promised him all of EY and yet, he didn't even have where to bury his wife. The Torah tells us the whole story about the sale to bring home this point.
R' Baruch Simon offered a similar pshat. He said based on the Malbim that the nisayon was in Avraham making such an effort to bury Sarah at all. For a non-believer after a person dies that is it, it's all over and the body decomposes and goes away. Therefore where and how they are buried is not important (witness all the cremations today). For a believer the story is much different. We believe in the eternity of the neshama and techiyas hameisim, after death things aren't over, the neshama lives on and will be one day reunited with the body. Therefore how and where a person is buried takes on much more significance. Avraham's nisayon was to see how hard he would push to get a suitable burial place for Sarah, would he show that he truly believed in these ikkarim of the eternity of the neshama and techiyas hameisim.
The Gra makes a fascinating diyuk. He points out that it says some combination of the words (in some form) קבר and מת seven times at the beginning of the parsha. 6 times קבר comes first and once מת comes first. The Gra explains that the 6 times refers to the 6 Avos and Imahos that are buried there. The word קבר comes first to signify that צדיקים even in death are considered alive. The 7th time is for עשו whose head was buried there. For עשו the rasha it says מת first because רשעים even when alive are considered dead.