Those links I used were through Buyee, which is the international bidding service that is hosted within Yahoo Auctions. You can't sign up directly to YAJ outside Japan. However, proxy services allow you to do this through them. Buyee is basically YAJ's "official" one. You can sign up and then you'll be able to bid, buy, etc. Payment via Paypal. They charge service fees, so expect a bunch extra on top, as well as shipping to their warehouse in Japan and then international shipping. I think there is a calculator in their system to estimate the final cost though.
There are other third party services that allow for YAJ bidding, e.g. Rinkya (which I used to use), Nengun, etc.
RHDJapan also has a sister business called BidJDM (check the icon on the RHDJapan homepage) that works the same way. They would be a good bet actually - RHD are excellent, so I have no reason to doubt their BidJDM service would be any different.
Here's a search for Longchamps via BidJDM:
http://www.bidjdm.com/bidding/search/result/?kw=ssr+longchamp&ci=
How to use BidJDM:
http://www.bidjdm.com/how-to-use