Keyboard shortcut to unmark all emails in Yahoo! Mail

1. Set your options (Gear symbol/Mail options) to show 200 messages per page.
2. In the search box enter a key phrase from the emails.
3. On the left side of the screen you can see a list of items you can use to filter the results (sender, folder, year, status). Sender Facebook might be a good one for you to select.
4. At the top of the list of emails just to the left of "sort by" there is a small box, check it. Now all the messages on that page are selected.
5. The buttons for "Delete/Move/Spam/actions" are now activated.
6. Move them to a folder. You can even create a new folder.
7. Repeat the last 3 steps.

I just managed to do this in a very round-about way on a fluke. Was so happy to have discovered it. Here's what I did:

1. Go to inbox
2. Click on "Unread" in left hand column
3. At the top, you will now see the number of unread emails and an option to "refine" your search. Click on "refine"
4. A box will pop up..here, you will change the search option that says "unread" to "inbox"...you will also change the Dates option to "custom" and enter in as large of a span of dates as you think you have in your email inbox (format yyyy/mm/dd)
5. This should pull up almost all of your emails and allow you to click the little select all box to "move" to transfer them over to another folder as you normally would for a smaller amount.

It may take a long while for it all to transfer and there may be a few hundred left in the inbox at the end which you can just transfer as usual.