I use a cheap foam mattress topper laid in the van bed with a thin piece of birch on top. Gives the machine a bit of a cushioning, allows you to slide the machine in on its back. Watch for power cables in the back, stick them inside if possible. And, as already said, have the seller open the machine and make sure nothing loose is in there.
Also, anything relying on gravity to hold it in place (bezel?) Should be removed before loading.