Technically they can’t be sold. The way around this is the broker purchases a ticket (or upgrade) for the “buyer” using your miles. As a result the miles you are selling have no shelf life and unless someone has an immediate need for the miles you won’t get a buyer. Additionally you need to have enough miles for the reward because the buyer can’t shop around for miles from multiple sources.
Unfortunately hardly anyone is buying miles because the airlines keep placing restrictions on what the miles can be redeemed for.