It might be that the seller has taken steps to remove their items and images from their website and cleaning up their digital footprint, and they’ve submitted a ‘right to be forgotten’ form to Google which has filtered through to Pinterest (Pinterest may have its own version of the form). The violation notice might therefore be automatic and not from the seller themselves.
I’ve had a few copyright violation notices from Pinterest when I’ve re-pinned artwork onto a mood board or saved an image from a site and then later on, the artist has decided they don’t want their stuff on Pinterest (perhaps due to its AI training feature, perhaps other people were claiming the work as their own or making collages or edits with the image without permission, maybe a gallery was cracking down on where images of artwork can be accessed, who knows).
I had a couple of stomach-drop ‘what have I possibly done wrong?’ moments at first but nothing further came of the notices. If you pinned an image from Folksy in good faith nine years ago, I don’t think you have anything to lose sleep over.