I guess one way would be to require the user to type in the price of the object they're reviewing, to seed the database... although that would only work well for a community with a lot of Android cell phones.
Now that I think about it, a database telling which products local people are comparing would be pretty valuable to local stores... They might be willing to trade pricing data for search data? Or a "featured" listing position?
Now that I think about it, a database telling which products local people are comparing would be pretty valuable to local stores... They might be willing to trade pricing data for search data? Or a "featured" listing position?