You buy a duped item, you lose. They do not return the money, the item disappears and if it really sucks, they give you a nice ban on top even though you had no idea.
THE PROBLEM? It's THEIR CODE that allows for duping. It's not our fault they can't keep their stuff solid so I have NEVER understood why we have to suffer for it. Buying multi billion Zulie items from 20 million is a bit different because it's kind of obvious that you're buying badness.
God though, you probably weren't around when Asian hackers duped so much that they were stacking piles of duped items quickly and fast so that they would make an NPC like storage go up into the air and unavailable to use because it was out of range. Zulie stacks were the same. A couple hundred zulie, one at a time and you couldn't use an NPC they duped under.