you should be able to bulk-edit the particular location field for all of your listings.
if you use the new my ebay format, go to "Listings -> "Active." Click on "Edit All x Amount of Listings" at the top of the page.
all of your listings should then open in a bulk editing tool and you should be able to select all of the listings in there and then at the top of the page select "Edit Field" -> "Item Location" and alter the item location for all of the selected listings.
if you use the old "My ebay Summary" it should still be the same kind of procedure, but instead of going to "Active listings" go to "All Selling" and follow the same procedure.
hopefully that makes sense and it works for you - i can get it to work on my computer, but there's so many interfaces being used by different people it makes it difficult to know what's where for each individual seller.
