Why does it say "v'Efron Yoshev b'Soch Bnei Ches")?
Rashi: It is written without a 'Vav' (implying the past tense) to teach that until now, Efron was an ordinary citizen. Now they appointed him judge in honor of Avraham, who needed him.
Malbim: He was their judge, and sat in the judge's place. This is why he himself answered.
Why does it say "l'Chol Ba'ei Sha'ar Iro"[?
Rashi: This teaches that they all abandoned their work and came to bestow Chesed to Sarah.
Malbim: Efron explained, I would explain the enactment like you want (refer to 21:8:1:3 and the note there), but Chevron is a great city, and all the surrounding people come. One cannot change the enactment unless the entire region joins, and this is impossible!
Avraham asked Bnei Ches to speak to Efron. We do not find that they did!
Rosh: The Torah did not record it, but surely they did, for he agreed to sell to Avraham.
Here it says that they spoke in front of "all who enter the gate". Why regarding Dinah it says "all people of their city"?
Rosh: Here it includes people who do not live in the city, but come for business. 1
Why does it say "b'Aznei Bnei Ches"?
Ha'Emek Davar: Bnei Ches, i.e. his family, hearkened to and understood his words. "In the ears" always implies like this. The others saw and heard, but did not understand.