It might be good to get the shop to pack it for you if you're unsure, they shouldn't charge much either.
When shipping, you've gotta be really careful, as the bike can be abused like nothing else.
Both wheels should be taken out, and spacers put in the dropouts. Pedals should be removed, same with bars, stem and seatpost. All this stuff should be padded up, and attatched securely. It might pay to remove the rear derailleur too.
Everything sensitive should be padded up, and then it all taped up tightly in a bike box. Especially since you're selling it, you don't want any chance of things to go wrong.