I have spent many weeks inputting content into a commerce system. The database is corrupt. (The website displays fine for an anonymous user, but if you log in you get a blank page. Backups/restore all yield same results) I have now set up a new version of the system, but would like to export/import content and content types from the old system to the new one. How can I do this? Which tables would I need to transfer?
The database isn't likely to be corrupt - unless you're coming up against a ridiculously unlikely scenario, it wouldn't work for any users at all if it was corrupt.
If the data was corrupt, you wouldn't be able to restore the backup, it would fail.
What you need to do is look into the server logs; you're currently seeing a WSOD (White Screen of Death), which is common in PHP applications.
Your server logs will contain details of the exact error your site is encountering, and you can use that information to decide on how to debug and fix it.