First, it is good Netiquette to confirm terms. So, do double check the terms of the deal. Given, things change.
Agreement Netiquette Classifieds
It follows, sellers change the terms of an advertisement without notice. And so, a bait and switch. Ask questions about the offer on the telephone to make sure that the deal is still the same as advertised.
Confirming advertisement information through a conversation gives a potential buyer more information about the validity of a post. The vocal variety, word choice, and explanations in answers to questions provide can give you a feeling about the honesty of the seller.
Confirmation through the telephone provides peer pressure for the poster to hold up their end of the bargain during the transaction. The potential buyer has the sellers word.
Confirm everything because terms may be different than you thought.