Try "automall" in the CLI. I haven't ever used it, but it looks like it does what you want from the help
edit: Maybe that undercuts everything, I dunno.
Is there a way to send all your items in you inv. into the mall at a price of 9999999999?
There is a way to automatically do exactly what you want.
Go to the general tab of your item manager. That is a list of every single item you possess which isn't in inventory, closet or display case. Highlight everything. (Use ctrl-A.) Then click on the "place in mall" button. All selected items go to the mall at maximum price.