Haven't tried eBay for a long time, but I have shipped a few saws 
without incident ...

Start with a flattened, rather large cardboard box.
Place the saw on the box diagonally, so you won't run out of real 
estate.
Crease the lines where you want the bends to happen, using double 
creases on each
edge to give the container a box-like shape.
This will be easier if the first two bends leave just a triangular flap 
against
the blade.
Make two more creases along the opposite edge of the saw.
Fold up a couple of filler pieces, two layers each, clamshell style.
Place the fillers along the blade's two faces and complete the fold.
Continue around & around, taping the cardboard along the way, until none 
of the box is left.
There ought to be excesses at each end; suitably fold them to protect 
the handle.
It helps to protect the handle if you make a double layer stirrup that 
fits through the
handle and can be taped to the cardboard; this will protect against 
endwise bumps.
At one stage I used inch-square wooden sticks to strengthen the box; the 
USPS has
been careful enough that they haven't been necessary.

Don't fret if the first box doesn't work out quire right; folded like 
that,
they fit the trash can rather nicely.

A big pair of tailors' shears, using the back of one of the blades, 
makes nice
straight creases.

George Langford, staying out of harm's way in SE PA