Mearat Hamachpela was guarded by burning fire

Medrash Talpiyot At Alef
-Before Avrahom bought Mearat Hamachpela anyone entering the cave would be consumed by fire.
-Certain places have different energies. Mearat Hamachpela was a major channel for holy energy. It was for this reason Avrahom chose it.
-Adom and Chava were already buried in Mearat Hamachpela.
-Since Mearat Hamachpela was a holy place it became a dwelling place for angels who would defend it.
-Any person not pure enough who would enter Mearat Hamachpela would be consumed.
-Efron and the men of Chais shunned and feared Mearat Hamachpela. They had no idea how to channel this energy.
-It is for this reason that Efron demanded such an outrageous, high price for the property.
-Meaning they realized the value of Mearat Hamachpela, they just couldn't put it to use