Why is the second "Efron" missing a 'Vav'?
Rashi (citing Bava Metzi'a, 87a): Because he talked big, and didn't even do little! Initially, he offered him the entire complex free of charge, whereas in the end, he took the full price 1 and made him pay Kantarin, which are accepted everywhere 2 (100 times larger than normal coins).
Hadar Zekenim: Without the Vav, the Gematriya is 400, the same as that of "Ra Ayin" (stingy), and the amount of coins he demanded. 3
Oznayim la'Torah: To understand the magnitude of Efron's duplicity, compare the 400 Shekel (even if they were Kantarin) that he charged Avraham to the 100 Kesitah
Seeing as Efron was still offering the property to him free, what does the Pasuk mean when it says that 'Avraham listened to Efron...'?
Rashbam: It means that Avraham took the hint and paid him the amount that Efron mentioned. 1
Malbim: Efron said 'bury your dead', i.e. and afterwards pay. Avraham knew that he would not acquire like this 2 , so first he paid, and afterwards buried.
This suggests that when Efron offered him the field free of charge, he was merely putting on a show, and that, in fact, he expected Avraham to pay in full.
What is the meaning of "Over la'Socher"?
Malbim: This was so Efron will not have any claim of Ona'ah (being underpaid).
Ha'Emek Davar 2 : They are bigger, for they are accepted in far away places where they do not recognize the mintage, and rely only on the weight.