Never sold a graphics card on ebay, have sold other pc part though.
Just be upfront and honest about the card, list as much details as you can such as the cards phyical condition (does it have rust or corrosion anywhere, if its clean or dirty etc), how old it is, what comes with it.
Pictures of the card will help.
The last thing you want is bad feedback or a paypal dispute over something silly that you have overlooked, 99% of the time you won't have any issues though.