If you are talking about regular gift stuff that people give for weddings, it brings to mind household items, like blenders, irons, sheets, towels and stuff like that. My first thought is that anything of that sort that is 12 years old has probably seen better days, so I can see getting rid of them for that reason, alone. She may be replacing them with newer items or something. If she had a yard sale and they were being sold there would it be wrong? They are her property, she has had and used them for 12 years, now she is recycling them by selling them.
If you mean a set of silver and things that a person would expect to keep throughout one's life as a sort of potential heirloom item, even though they are her property, she could be a bit more discreet than Facebook.
But the bottom line is that they do belong to her and she can do as she wishes. If you have an attachment to something, maybe you could purchase it for your own use. Otherwise, I would just let go of the situation and move on.
I would probably be feeling a little bad about it, too, though.