They can probably be used for giftcards etc via the MileagePlusX program / app.

It's against United's terms to sell your miles.

Not that this stops it from happening, Google eg. "sell frequent flyer miles". It's doubly risky though: from what I can tell all the companies in this space are by definition more or less dodgy, plus the airline will catch on pretty quick and confiscate your miles (yes, they can do that) if you start redeeming first-class tickets for a whole bunch of "cousins" with random names and in random places.