Cash is the only safe answer. Everything else can be reversed or manipulated. $6000 cash can fit in a letter envelope. I sold a car for $8000 and received cash.

If you can get the item back on a pallet, strapped down, it can likely be shipped for $400 or so by companies like Estes. They would need to be able to get a big truck with a liftgate near the item and roll a pallet jack on a smooth surfaxe like concrete. I would make the buyer arrange their own shipping though, let the liability be on them.