Thinking Ahead with Millions of Users

A friend of mine and I had the idea of creating a new type of classifieds site or something that would attract thousands of users in the first year or so, and upwards toward a few million.

Users would not have their own blogs, but rather just create a "post" with their item to sell. Meaning millions of posts.

We're wondering how this relates to Multi-DB and how big we need to scale our MySQL databases to handle this load.

It seems like Multi-DB is primarily for having thousands+ or millions+ blogs with each on in a separate database rather than just tons of users and posts, correct?

Also, if you don't think Wordpress MU is the best solution for this, do you have any other suggestions?