Very hard to give an opinion without knowing a) the specific card and b) the amount of cash you will be getting. But I will try
My concern is when you say "In this case, the item in question is one of the Crown Jewels of my collection. I paid more for it than I’ve paid for any other single piece. And at the moment, it’s arguably the most valuable piece in my collection." I think you have your answer there. Keep it and enjoy. And maybe let go of another piece if you want the cash.