If you have to ask...

To each their own...Ethics-wise, yes, it's unethical because you've received full value & benefit.

Reality-wise, if they failed to collect it (and you, in good faith, were willing to give it up), then feel free to use it to your advantage & benefit, even if it means trading it. Sometimes GA's don't collect coupons - I'm guessing you're speaking of an upgrade coupon, but how many times have you ever ordered a pizza or something & not actually given the coupon? Or used a coupon on an car rental & never actually physically handed it over? That's no different between a coupon for a 9.99 pizza & a SWU - an uncollected coupon still has value. I'd use it.

The reason many companies issue coupons is because it encourages you to use their products. Here's an example: I had a choice between flying on AA from the US to Europe for 500.00 & an I had a coupon for a free Upgrade & flying on UA from the US to Europe for the same price & didn't have a coupon for a free Upgrade, I'd fly on AA - thereby giving them the revenue that if all things were equal, UA probably would have gotten from me.