Did you offer any sort of returns policy to the buyer when you put the item on e-bay? I used to sell alot fo stuff on e-bay and there are some real idiots out there!
Try and maintain a good communication with the buyer if you can, to try and avoid negative feedback. Ask them how the item differed from what they were expecting. Ask them to look carefully at your listing and explain what they mean about it being different. Then you will be able to deal with the problem. If they are just being an idiot, then you should be able to get some assistance from the sellers guide on e-bay. Also, was your description in the listing absolutely accurate? If you stated any faults or that it was "sold as seen" or "no returns or refunds" then you are probably OK, but sometimes it is easier just to get them to return the item and offer a refund (don't give this until you have safely received the item back in its original condition). Then you can just relist it and cut your losses! You can also block buyers on e-bay so they can never bid on your stuff again, as you do get some real time wasters out there.
There is lots of help and information for sellers available on e-bay, so I would advise you to use these resources when dealing with this.