Do you have a link? I couldn't find her costume on ebay, but I did see it last Friday, maybe not the same one, but one on ebay.
Was it her pants? I found pants worn by her on their
website.
Background on the seller.
http://www.unitedmoviecollectorsguild.org/aboutme.php?userid=100088&ame=1
I'm assuming they bought the pants from Premiere Props, as to why the photo of the pants still has a logo of Premiere Props on it. If you check their
ebay and click on their listings you'll see a lot of items were from Premiere Props they re-sold, and with their good feedback, people have claimed they received items from them.
Premiere props, who is for the
most part is respectable on authentication, also handled a lot of the Underworld props and costumes. You could email the seller, as from the bio seems like he would be nice and get larger pictures so you could screen match. It looks like there might be marks on it to do it and he listed which scene it was used in. I'm assuming you would get the Premiere Props C.O.A that came with the pants like stated in the bio.
Somebody will probably post something soon about them.