I've used eBay for all my Double Dip replacement situations (T2, Boogie Nights, 2001: A Space Odyssey, The Professional, Pulp Fiction, Swingers) I try to sell them before the new verson of the DVD comes out.
I tend to use Amazon Marketplace (creating a seller account is easy; they deposit the money you earn into your bank account--after their cut--and compensate you for the shipping charge).
Both Digital Eyes and DVD Empire buy & sell used DVDs (though neither will pay you a great deal, at least you don't have to wait around for a seller & mail out a bunch of different packages). If your community is anything like mine, a lot of local stores have sprung up that will buy & sell used DVDs, too.