Is there a way to raise all prices by a certain percentage or amount at once?

As SegundaVez says batch edit (which is called Update Items as a Group)

Item Basics
Select Set Price fill in box
Select Item(s) to change
Click on Apply Changes

If you would like to edit items individually

Upper left corner
Sell on Bonanza or use drop down menu
Add or Edit Items
Mouse over item and

Edit
Delete
Duplicate

select edit
Scroll down to bottom
Change price
Save Changes

Once done making all your changes, hit the Update Booth button (not really necessary, but it does kickstart the google feed to send updated information)

For jenrun

To change prices by a certain percentage
Update Items as a Group (Batch edit)
Item Basics
The box at top…over to the right

Select
Apply [URL removed] %
Fill in discount
Select Items
Select Apply to Items

reversing the discount [URL removed]

percentage_to_change_by = (desired_price – current_price)/current_price

In the case of making a $9 become $10 again,

percentage_to_change_by = ($10 – $9)/$9 = 1/9 = 11.1%. or putting .111 in the apply discount box

Always test your figures on a few or one item to be sure it is doing what you want it to do

You CAN raise all prices by a certain percentage.

1. Goto “Selling” → “Items” → “Batch Edit” → “Item basics”
2. Use “Apply discount” field with NEGATIVE discount % value
3. Select required items and press “Apply changes” button.