Haute Club Membership
The Haute Club Membership Card is a completely worthless item, but it sells. It can even sell for crazy marked up prices. I have no idea who buys these pieces of junk. There is no in game use for them. I found a few stacked in a freshly purchased guild bank. So I tossed them on the auction house and they sold for around 10g each and sold within a couple hours of posting. All of them sold.
Haute Club Membership Card Drops
The Haute Club Membership Card only drops from some of the mobs in The Plaguelands: The Scarlet Enclave, which is the deathknight starting area. The Citizens of New Avalon and the New Avalon Councilmen are the best sources, but Mayor Quimby, Scarlet Commander Rodrick and High Inquisitor Valroth also can drop the membership cards.
I don't recommend farming these, but if you happen to get some while doing the deathknight starter quests hang onto them. They are very helpful for getting starter cash for your new deathknight. Just post the membership cards onto your auction house. If you see one for a few coppers on the auction house, be sure to snatch it up for an easy flip. Again, no idea who buys these things, but they do sell.