People are buying these DVDs because they have no other way of getting them. Torrents are a poor way of sharing files because they are hard to find. Even when a torrent is found, people don't neccesarily have access to it. For example Demonoid at the moment won't allow new users. At some stage new registrations will probably be made available. By then the torrent may be dead or, for some, their time is worth more than their money so they will just buy it.
Torrenting this DVD will not have a significant affect on the eBay sales.
If someone shared this with eMule it would take on a life of it's own. eMule has built-in global server searching. It would not have to be seeded for more than a day or two initially & then will not die unless a superior version becomes available.