I run 2 iPhones with the Ebay and Paypal apps on them. The apps are small, can do updates via 3G Network (no WiFi needed). I know because I updated my eBay app at my kid's footy training last Wednesday.

16.9MB - see http://mobile.ebay.com/iphone/ebay

Maybe the person has an Android phone? But nevertheless, they don't need to use an app - they can use the web browser in the phone and use the mobile version of the standard ebay website.

I honestly think there is more to this than meets the eye.