At my sister's wedding there was multiples of everything, some of it never got used, some of it came without tags or labels so she didn't know who things came from.
In her divorce, she got a lot of their wedding gifts because her XDH had enough stuff on his own to furnish his own place and didn't need any of it.
Since then, sis has had a child and moved several times. She's celebrated more milestones. Her wedding was less than a decade ago, and I'm sure there are some things that if I asked her where she got them she'd remember they were wedding presents and who got them for her (very high-end things with a specific pattern on them), some that she'd know were wedding gifts but not from whom (other assorted kitchen items), and some that she wouldn't know whether or not were wedding gifts or from some other occasion, whether or not she could identify the giver. There's one waffle iron she has that she insist was a wedding gift, except I specifically remember picking it out for her for Christmas one year. It makes oddly shaped waffles. She did get two identical waffle irons for her wedding, so I can see the confusion.
OP's cousin honestly may or may not remember where she got the gifts and who gave them to her. She's moving on with her life and is giving her FB friends an opportunity to buy from her anything that she's willing to part with. If there's something that another of her FB friends has been interested in for a while, she's letting everyone know that it's open market.
I see it as boiling down to this: OP remembers this gift clearly because she had a lot emotionally invested in the gift (went in with another cousin on it, it was for a wedding, it was an important thing. Cousin selling things may have less invested in it. It was one of several wedding gifts, the wedding has dissolved, depending on the gift it may be sitting around not being useful whereas the money or space obtained from selling it could be very useful.
OP, I can understand you feeling a bit hurt over this, but I wouldn't bring it up. I wouldn't dwell on it. You don't know cousin's situation fully, but I honestly don't think she means anything by it.