It is common for instruments to have limited legal language but the Marriott T&C are public and are the governing rules.

Selling them is wrong and the more that it happens, the more that companies start putting more restrictions on these to make them harder to use.
Right now they are easy to give to family or friends as a present. If there starts to be misuse like this, then those will stop.

Don't do it. It hurts all of us.