Implementing an offline Inventory that must sync with a server

My next game will have a substantial inventory system and must both be playable without a internet connection and be able to sync back to the server.

I have SQLite up and running for iOS and the basic tables etc are built. What's the best way to merge or compare data when doing the sync? Assuming that the player could make changes on both devices. Asking them to force a sync at the start of each session is not desirable.