It happens to all of us eventually (except for the Mac collectors). At some point, we decide that it's time to take that old Mac that we've just replaced and try to get a few bucks out of it. Some of us donate our machines to schools or other worthwhile organizations, and if we have really old equipment, we may just recycle it. In any case, there are some things you'll want to do to prepare the Mac for sale or recycling.
Step One: Make sure everything is backed up
There's nothing worse than nuking the hard drive on a computer and then realizing that you've just destroyed irreplaceable photos, documents, or recordings. Even if you're using Time Machine