A button to delete all ended auctions would be convenient. What does the button labeled "all" do?

That feature was implemented already. You can find it in "Tools" named as "Delete Ended".

I don't see "all" button. Where did you find it?

It is in the center of the screen, at the top, just under the dark blue bar with "add item" and "update status."

wrote:
It is in the center of the screen, at the top, just under the dark blue bar with "add item" and "update status."

That's a pager. Since you set your preferences to show all items on your page -- you see "all".
If you put something like "10 items per page", you will see "1-10, 11-20", etc.