Now, i want to blame shame on people scamming others and to repeat some important basic rules to help people not feeling like me now :
1 : Enter the name of the person you are dealing with in the search function of GOG forums to check on their forum posts / rep.
2 : If there is none or really few, and / or that user is quite new or even unknown, it is more than likely a scammer so let them go first in whatever they offer! ALWAYS!!
3 : If it is a known user, lowest rep and post activity person should go first in the trade.
Good list, but one more thing one should do, when checking the users rep and date on the post, is to click on the pm button next to it to check that it's the same user one has been talking to via pm. This is important because trusted users with good rep where impersonated before!