What do you think of the value of these headboards and foot boards?

Two twin's next to each other is the same width as a king size (72"), so these could be configured as a single king bed. Twin rails are shorter than a king rail, but you don't have rails anyway. That said, I doubt it would be worth finding the parts and putting together rails that work if you were to looking to sell. Only worth the effort if you want them for your own use. Agree on first half 20th century date.