If I buy a game with a "non-tradable" coupon, is the game tradable?

If I buy a game with a "non-tradable" coupon, can I trade the game? E.g. can I technically buy the game using the coupon, and then sell it on ebay?

You can not trade or sell games bought through the Steam store at all, regardless if you use a coupon. If you use the coupon and select "Buy as a gift" (instead of "Buy for myself"), you can gift the game to one of your friends. This feature does not provide any features or safeguards for a trade - it's a gift, meaning that you give something and get nothing in return.

Selling your games, if you found a way, is also against the TOS of Steam and will result in the shutdown of your account, and the loss of all licenses to games you previously owned. (emphasis mine)

You are entitled to use the Content and Services for your own personal use, but you are not entitled to: (i) sell, grant a security interest in or transfer reproductions of the Content and Services to other parties in any way, nor to rent, lease or license the Content and Services to others without the prior written consent of Valve, except to the extent expressly permitted elsewhere in this Agreement (including any Subscription Terms or Rules of Use); (ii) host or provide matchmaking services for the Content and Services or emulate or redirect the communication protocols used by Valve in any network feature of the Content and Services, through protocol emulation, tunneling, modifying or adding components to the Content and Services, use of a utility program or any other techniques now known or hereafter developed, for any purpose including, but not limited to network play over the Internet, network play utilizing commercial or non-commercial gaming networks or as part of content aggregation networks, websites or services, without the prior written consent of Valve; or (iii) exploit the Content and Services or any of its parts for any commercial purpose, except as expressly permitted elsewhere in this Agreement (including any Subscription Terms or Rules of Use).