Congrats on your win!
I hate to be a killjoy, but make sure the seller packs it well so it doesn't get damaged in transit.
If it is in it's original box AND packing material, great, just have them double box it and stuff styrofoam sheets (not pellets) between the boxes.
If it's not in the original packing, make sure that it is packed securely. Bubble wrap wrapped around the arm to the point that it is stuffed into the box. Every open area of the box has to be filled with either bubble wrap STUFFED into the crevices, and/or folded cardboard so that it absolutely cannot budge inside the box. Then double box it with the styrofoam layer as stated before.
Unfortunately the delivery system tends to toss packages around like they're bean bags, so pack it like you expect it to drop off a 10 foot conveyor belt. Then, it should arrive intact.
Have them pack accessories in another box that can be put in between the double box.
Shipping a machine just in it's case, then in a box is not sufficient.
Check out the Vintage Sewing machine shop if you need photos and horror stories of machines shipped from eBay sellers.
Dictate exactly how you want it packed. DOn't assume the seller "knows how."
IMHO