You could try selling it on craigslist. But be patient. It takes time before an interested buyer will contact you, especially for such a unique item.

It's only worth what someone will pay. We had intention if using an old mantle and surround when we build our fireplace. It cost a little chunk of change. But we found out from a few fireplace manufacturers that we cannot use it due to the fact that the opening is too small for modern fireplaces. One even explained to me that if you look at old mantles in older houses you can see where the heat damaged the wood. That is how a lot of older houses burnt down.
As far as your mantle, it is unique. To someone restoring an older home it may be what they are looking for. Craigslist would probably be your best bet. Take some good pictures and advertise the price that you are looking for. Like Haus_proud stated though, you may have it for awhile.
On the other hand, it's paid for and served it's purpose. You may have to practically give it away if you don't want it taking up space. You can't always get premium money for an object. Sad but true.