Just contact them
When I had to get my Touch cover repaired, used the website to arrange a callback from MS. Within 5 mins a guy from MS Ireland called me, and they sent a package for me to send the broken one off, at which point they decided to replace rather than repair, so sent me back a new cover.
Bear in mind if you do this, you will be without a charger for a few days. All in all from sending it to them to getting a new one took me around 5 days I think.
P.S. If you are worried about the warranty thing, just don't mention eBay
. I was never asked where I got it, all they need is serial number, and if on the system it shows that it is less than a year old, they should provide you with all the help you need.