I sold and shipped a sub several years ago that weighed approx. 50lbs. Not terribly heavy compared to the Fathom. I did not have the original box, so I used a box I found at work. I bought a pack of styrofoam insulation from Home Depot. There was about 6-8 sheets in the pack and that was enough to line the box. I added styrofoam peanuts I grabbed from the shipping dept. to fill in the gaps. The sub was delivered in perfect condition.
With yours, I would try and use the original packing since you still have it. Reinforce it where needed with strong packing tape. If the sub is too heavy to handle, flip it upside down. Lower the packing over the sub including the box, then flip the whole thing right side up. That way you are not trying to lift the sub's weight, but rather simply "rolling" the sub. State in your post that you can either split shipping with the buyer, or have them cover it. I'm sure there is a lot of demand for that sub, so a bargain price will leave room for the buyer to pay some of the shipping.
There is a site I just sold my Axiom QS8's through. Free listing. tweakcityaudio.com
Good luck!
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It's all about the hardware!