Marta, can you ask the seller to try to find the bobbin case? Maybe they had another machine in the house, and maybe they have a Sewing Box. If the machine was indeed used in the household there, then they likely have it in a Sewing Box and just don't know it.
This happened to me with a machine. Often the family selling it just does not know the difference, what is meant to go in the sewing machine and what is just sewing machine junk. Maybe worth a try...?