Do I have to use the app (Givi) to bid?

No, while it is convenient to use the app, it is not required! You can also use a desktop or laptop computer.

I've registered for the event, now how do I bid on items?

Once you've registered, a unique code will be emailed to you. Use that code to make a login using the auction website or the Givi app. You will need to be logged in (using the login you make with that code) to bid on items.