It seems crazy you could get banned from a traded steam gift. How would anybody really know when its legit to trade? I can understand if you obtained a key from an incorrect region and used vpn to activate. In this case you are obviously working around the system and therefore accepting the risk. I just don't see how steam trading could be fairly policed
Because it is crazy. And unfortunately Steam's TOS gives Valve broad powers in determining who is violating their rules. I have read cases where people have been banned and asked for an explanation and received nearly none.
Not all regional keys require VPN (very few do, relatively) but their origin is obvious. Publishers keep track of which keys get issued where if they care to, we know this because a game like X-Com seemed region free, only for people to find out later it was region-locked. And Valve obviously knows the source of every Steam gift. That being said, they really don't do much to prohibit regional trading between users.
In my own opinion, I think a few of these per year are normally okay, unless Valve is doing a ban in waves for regional pricing offenders (which they do sometimes, either to make an example or to people who severely abuse the system, who knows) and some unlucky people get caught up in these.
But as someone else said, our regional sellers are honest, and people who buy from them know that they are taking on a certain level of risk when they do (however minimal, it is there).