Jennie,
I didn't realize that you were looking to sell your existing ones; I thought you just wanted to buy another one! Why were you going to replace it in the first place? Just because someone told you that mattresses need to be replaced every so often?
Initially we used the two horse hair mattresses as a king sized bed. Then we reconfigured the master bedroom to make room for more closet space and purchased a new queen sized bed. That's what needs to be replaced.
One horse hair mattresses and box spring was then placed in a spare bedroom/den. The other was put in the attic. Neither were used much over the years.
When I was recently in a high end carpet and furniture store browsing for a new queen size bed, I jokingly mentioned about our old horse hair mattresses, and the salesman surprisingly knew all about this type of mattress. He is a history buff and he told me how royalty of many countries valued them and passed them down as family heirlooms.