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?

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