If you want to be extra safe you should take out any expansion cards (although if you're selling it on eBay you should sell any expansion cards separately anyway for the best price).
Also take out the motherboard and place it inside an anti-static bag, wrap it in bubblewrap, and then place it inside a separate box (at least 2 inches bigger on each side than the motherboard itself) and then fill the extra space with polystyrene chips. Put the screws in a little plastic bag and put it in the box with the motherboard. Then you can place this box back inside the tower case itself, secure it with tape and put the whole thing inside a large strong box again filling any extra space with polystyrene chips.
