But...it is common in the airline industry for agents to purposely fail to collect stickers/certs for upgrades. There was one US Airways agent at MCI always took one 800-mile certificate from me for MCI-DCA, a 930-mile flight. I always offered her two because I didn't want to take it for granted, but she always tore one off and handed it back. If I only had one cert on me I wouldn't request an upgrade...kinda felt guilty about it. Anyway, I don't regard failure to collect certs as stealing and I don't think that's just my human mind sinking to a low, low level. It's the way the airlines do business. (On the DCA end, they took two certs for the same 930-mile segment. CLT demanded two certs on an 807-mile segment, so I know there is no formal grace period built in to the certs.)

If you are lying about the certs you have or otherwise trying to game the system, then yes that's a different story. But in the OP's case, we don't know whether he was actively trying to game the system or exploit a loophole - or if the agent simply didn't take the cert from him.

The second part of the post - selling certificates - I have no problem with. I know the airlines do not like it. I do not sell my certificates, points, or miles because I like to use them. But I don't have an ethical problem with people who do, as long as they are candid about it. If somebody had a cert I needed, I would probably buy it off of eBay if I felt that was a good value.