Well,
in theory you won't need anything else but the card itself. Just make sure the seller has good feedback as a seller and that he is going to pack the card well so it doesn't get damaged during transport.
But again I am saying in theory as there are many contributing factors that can play you a bad joke.
Make sure the PSU you have will provide sufficient power to the card.
Make sure your motherboard has the necessary slot - whether it being PCIe, AGP or whatever it might be. It's hard to predict what can go wrong with so few details but if the card you are buying is similar to the one you currently got you shouldn't have to worry much.
I don't want to confuse you further.