One thing to consider before trading or giving it away. Does the coupon in question have a number or something like that to identify it and you gave that number when making the reservation. If they failed to collect it and then you give it to someone or trade it to someone and they try and use it and they are unable to as the "number" on the coupon shows it was used you may be in hot water with whom you gave it or traded it to.

If no such number or identifying marks exist then you were probably comped. So use it again or trade it or whatever. Just make sure you don't violate any rules or conditions on bartering or selling said certificate.

Use your best judgement and let your conscience be your guide.