Sorry to hear of your trouble, Yohann. I would be inclined to say it’s not wholly on the seller. Having agreed to f&f makes it go from a transaction with buyer’s rights to a gentlemen’s agreement. I’m assuming this was sent from outside the EU, otherwise what reason to lowball declared value? Or does it make a difference in terms of postage and insurance? At any rate, both seller and buyer would be wise to make explicit all needs, wishes and details, so one is on same terms. Something to learn in this for everyone, sadly at your expense.
FWIW, I’ve often, prior to sending, stated a 50-50 liability in case of loss of parcel (using untracked, but that's beside the point) and think that is fair. Unsure what I think would be fair in this case, as in who is responsible for it once actually signed for and delivered, which in principle means it is out of the hands of the seller, but seller should clearly have asked about value declaration before shipping.