alt Hacker NewsSlashdot/Reddit/Facebook etc all build communities, when they get too large they splinter until they become increasingly specific. You've done the opposite. I think you are going to need to group common products, then gradually break it out. ie, food -> deserts -> cakes -> chocolate cake -> chocolate cake from <shop> etc
That's how I pictured it, users scan all the bar codes they have access to and it builds similarity profiles.
I pictured scanning all my CS books for example, and I'd probably end up in a group with everyone from my class at school who did the same, for example.