Just as a note, some boxes like that will have sellable contents (like stuff from FUN scratches) even if the box itself isn't. But since that came from Fresh Finds, they won't be sellable.
@Plutia-Chan said in What does the blue x mean in this pic?:
And can I hand it over to my 2nd character once I unseal it?
You can transfer outerwear/outfits and the tickets for the basewear/accessories between characters, but once you actually register the basewear/accessories/etc they'll be stuck to that specific character. So just make sure you know which one you wanna use the set on before using any.