I want to bid on an ebay uk item that I've listed in my ebay (USA) watch list so I can use my ebay bucks. Will mybidder bid through the USA ebay site?

By default it will always try to bid using eBay US. If the item is not available on eBay US, it will switch to eBay UK (these are mostly items which do not accept PayPal payments).

If you can open the item using ebay.com, it will snipe using eBay US and so your purchase will use eBay Bucks once you completed your payment to the seller.