I would suggest it's worth a fair bit more than what you're offering it for.
But yes, don't entertain any ridiculous low-ball offers from FB Marketplace. The clowns offering £500 know they'll make more than that back from just the Cat and its precious-metal goodness.
Good luck with the sale, I would have snapped this up myself if all my money wasn't invested in nappies, wet wipes and toys for my son
Cheers