GT Legends Steam keys are currently FREE for anybody with a Polish IP address, so I would not recommend trading them.
I'm quite sure that it's pretty much illegal what you're doing here since the codes are actually meant for people who bought the magazine. It has happened to me once that I bought the mag for a full version, didn't instantly get the serial and when I tried a couple of months later the service was out of keys because some assholes on the net had spread information on how to get it.
Not implying that you're an asshole. I guess you weren't aware that this serial is actually meant as a full version for people who will buy the next issue (apparently the site administrators unlocked the serials before the issue that GT Legends will be included in even hit the stores).