Best way to sell an engagement ring?

My brother was fully ready to propose and build a life with his girlfriend, ring bought, everything planned and then found out she was cheating. He, of course, wants nothing to do with the ring itself, gave it to me to deal with and just wants to recover some of the money spent.

I want to do the very best I can for the guy and am wondering what the best options are around jax or online for a situation such as this.