I think a mobile app should not be as full-fledged as the actual website. I can see only needing the following functions.
1. Login
2. View collection
3. Add/remove item from collection (including price, data, condition)
4. Search items
That's probably it. I wouldn't worry about adding the ability to add/modify database items or uploading pictures. Maybe if someone finds an error with a particular entry, there can be a button to report a problem, then have that report go into a queue that the DB mods can check out.
Another consideration is just how fast the app would work. That being said, probably should only use thumbnails for displayable images?
Well those are my thoughts on the subject.