I repeat my suggestion from another related thread -- if UA takes it so seriously, they ought to publish / make a bit more clear to their customers that selling is not allowed.
The onus is 100% on the customer.
At some point, people need to take responsibility for themselves and not expect the world to be spoon-fed to them. United publishes the rules, and if people don't want to take them seriously, they never will.
You can make something fool-proof, you just can't make it damnfool-proof.