No. Technically, it isn’t illegal to sell your miles (except if you’re conducting part or all of the transaction in Utah). It is against the airline’s frequent flyer contract to do so. Some people attempt to sell their miles through a miles broker. We don’t recommend this because the potential risk is not worth the extra income.
Airlines are always on the lookout for people who do this. If they catch you (and they likely will), they’ll confiscate all of your points, not just the ones you are attempting to sell. Trying to get your points back is an exercise in futility.
While you can’t sell your points, you may be able to gift them, pool them, or transfer them. Some airlines may offer you the ability to buy someone else’s ticket using miles.