How can mobile tickets (on smartphone) be resold at  the venue ?

I don't think they can. One of the benefits of this is it prevents scalpers doing this.

They can’t be. 

Part of their purpose is to prevent ticket-touts/scalpers.

Depending upon the terms and conditions and time limits, you might be able to get a refund and then the official box office can resell your place at the event at official prices.

All depends on the terms of the ticket and the venue. Terms may vary from one event to the next, so always best to find this out BEFORE buying tickets, especially if tickets are being sold through more than one source. If venues are not selling tickets, but 5 different sellers are, you will have to deal with the seller you bought the tickets through. 

The buyer gets a fresh barcode and the old one gets burned so it won't scan.