If the iPhone app is anything like the Android app, it is effectively useless.

I would question why the seller can't find a real computer. That's what I do. I will, occasionally, go to the classic (real) site, but it's a PITA to use a full computer screen on a phone, and the mouse hover dropdowns don't work that well without a mouse.

Maybe the app makers should have a think about what needs to be provided.

Until then, advise your seller to get themselves to a computer, not a phone.