Just un was banned for something people were doing months before he did it, N4n0, bsgpseller, and a few others all sold duped gold before just un discovered his dupe, karl was curious as to how he could be making bills whereas the crucible dupe could only make a few mils. I'm sure karl's personal financial situation had something to do with his ban as well but the reason he was banned was for calling karl a lier, originally he was given a twc + market ban but then karl started saying stuff about him which weren't true so he pmed karl and karl banned him.
It's more about the double standard that's set, the matkias guy offloaded billions of gold to resellers here and was never banned. The resellers never admitted they sold duped gold but if you look under just un dudes or matkias' PA/ sythe vouches you can see how much gold was offloaded to resellers, Grave even offered to buy the duped gp at a discounted price but never stated in his sale thread he was selling duped gp.
The whole buying and selling stolen gp thing didn't promote due diligence in companies all it promoted was to be ignorant of the fact. I'm sure gold buyers still buy stolen gp considering the mark up you can put on it, all they have to do now is say they didn't know it was stolen and they'll be fine.