Oh, absolutely. Trusting days are over.
You can mail the support or go tho IndiGala discord to ask.
But before cutting ties talk with the user, maybe he made a mistake and is ok to fix it.
I can talk my case, the first two trade i made where for two used keys (i was not able to check them becouse i already owned the game), but efter activating one of the won game successfully i was able, throught the helpdesk, to contact the user and give him a key replacement (i got negative feedback but i was ok with it).
The other trade was a fake so i have not fixed that error (it was a trade for Battle for the Galaxy, owning the dlc Ice Bastion Pack i wanted the game but i got scammed with a second dlc).