Why does the Pasuk repeat the Isur of Ona'ah again?
Refer to 25:14:3:1.
What are the ramifications of Ona'as Devarim?
Seforno: It pertains even to Ona'ah of words, Geneivas Da'as 1 and advice - even if one gains nothing by doing so.
And why does the Pasuk add the phrase "Ki Ani Hashem Elokeichem"?
Seforno: 'I am the G-d of both the buyer and the seller', Hashem is saying, 'and I am particular about either of you being hurt'.