The trouble with trying to diagnose where the problem lies is that despite your apparent prior success with it, the app is known to be notoriously buggy, yet by the same token, it is also true that eBay has been experiencing some really peculiar intermittent postcode-related issues as well, and the fact that the app worked OK on your iPad could indicate that your problem was actually with eBay rather than the app.

It's difficult to prove either way, but as soon as the app worked on your iPad, I'd have been inclined to immediately attempt listing again on the iPhone. If it once again failed to function properly, I'd be inclined to blame the app, but if it worked OK this time, I think I'd be more inclined to suspect that eBay was again experiencing one of its intermittent glitches. It's nowhere near definitive proof, but the results may help you to determine whether the problem was with ebay, or the app itself.