My iPhone app requires sellers to verify their email address and phone number, when should I do that?

I searched User Experience before posting this question. I saw a lot of related questions but not this one in particular. My organization is creating an app that permits users to sell a service or a good. The user downloads our free iPhone app. Before they can proceed to use the app, before they can do anything at all, they must create an account.

We permit oath through Facebook (permitting them to create an account with a single button press).

We also permit them to enter their name (first name and last name though we are thinking about using just one field for the user's name), we require them to enter their email address and their phone number (for SMS).

My question is: When should we do that?

My gut instinct/intuition is that the least annoying thing to do is to ask the user to verify their email address and phone number right away, immediately before we let them do anything. We are already requiring them to make an account -- why not immediately require them to verify their email and phone number before we let them do anything, even access the account.

Another member of my team feels very strongly the other way. He feels that we let them create their account (either via OAUTH via Facebook or by entering in their email and phone number), but that we do not require them to execute verification until we need it, like when they go to create a posting for the good or service they are selling through our app.

Presently there is no version of the app for consumers, just sellers, sellers use the app to communicate directly with consumers who get text messages, etc. when they sign up with the given seller (believe it or not this method and methodology works as our sellers are grateful to have our platform for delivering their goods and services and getting paid for them.)

So, should we:

A. Execute email and phone number verification right away and require that they perform email and phone number verification right away.

OR

B. When they create the account we send them email and phone number verification but we do not require them to complete it until we need it -- like when they go to make their first posting.

I like requiring it right a way for a variety of reasons, including, frankly it is easier and less complicated to do first before we let them into the app.

But is it a better user experience if they can setup their account right away, get the verification right away, but do not haver to complete it until we need it (like when they make a posting)?

Help with our design issue is greatly appreciated.