[Chad]

Here's what happens.
Scenerio 1: A new user registers on your forum. Good. He's ready to post messages.
That same user decides to pop up a classified ad, he goes to the classified site and logs in using his forum credentials. Then he updates his member information to include his city, state, address, etc

Scenerio 2: A new user registers on your classified ad site. He's ready to post classified ads.
Same user visits your forum. Logs in using the information from the classified ad site.

Scenerio 3: A user on your forum edits his email and password. He has to use his old email and password to log into the classified site unless he updates it on that site as well.

I cannot stress this enough: This is a user LOGIN and REGISTRATION sharing system, not a USER sharing system.