The pentagram is a five pointed star. When enclosed within a circle as it often is in pieces of jewellery it is referred to as the pentacle. The words in modern usage are often used interchangeably and the association with both the craft and the occult is present in both.
I’ve got four separate suggestions and I’ll list them for you.
The pentacle has strong associations with protection and many craft practitioners will wear the symbol as an intended source of protection. This isn’t the sole reason for wearing it, sometimes it worn as a statement of belonging to a craft path and other times it may be a subtle symbol worn to try and meet others on the same journey. The protection association however could be relevant in the sense that the woman wanted you to wear it to offer you protection against something. This might be likely if there is something in your life currently causing you distress or a situation you are already aware of that you may need protecting from. Possible the woman picked up on this and offered you the pentagram/pentacle as a form of defence.
2. Object she wanted to get rid of
I’m not suggesting the object itself has negative associations but it might represent something negative to the woman. Perhaps she tried her hand at the craft path and it didn’t work out for her and she wanted to rid herself of the symbol without actually destroying it. If this is the explanation then she’s probably just given it to the first person who came along to her garage sale and there is no significance in the fact she picked you. (Also possible that you look like the kind of person who might appreciate a pentacle? Do you dress in the style that would suggest you would get some use out of a pentacle?)
3. Reminded her of a family ancestor who owned it
I wonder if you might remind her of a family ancestor or somebody in her past who the pentacle would have been of some significance to. Possibly she recognised you from a past life even and wanted to make the connection with you by gifting the pentacle.
This explanation wouldn’t be consistent with her having an item she valued on her at a garage sale (unless it is something she herself was wearing herself and took off to give to you.)
4. Recognised you as a witch and wanted to point her in that direction
This explanation to my mind is the most likely of my four suggestions. She saw you as somebody she believed would make a good witch, possibly recognising something of herself in you and gave you the pentacle to steer you on the path. It is always hotly debated as to whether a witch is a person who becomes a witch or a person who actualises the natural witch within themselves. This would fit with either explanation.
In terms of the witches’ ball I’m thrown by this. I can assure you there isn’t an annual gathering of witches - and if there is my invite has certainly got lost in the post so I might have to pull a Maleficient there!