Okay, Let's sort this out.
You cannot under any circumstances buy a qi restricted set from another player if you don't have the qi. Not through the exchange, not through the shop, and not through an admin. The people that do have the power to transfer the qi-restricted items are under orders not to.
In the past, for large events such as the clan league, you were able to win a demon set. Most of the demon out there today is from these clan league tournaments.
In the past, mainly before 3.0, the rules on transferring these qi-restricted items were a little less strict, and it did happen from time to time, which is where some of the other demon/pure bits come from, but that doesn't happen anymore.
In all honesty, you can get your hands on whatever items you want, pure, demon, elf, anything, if you're willing to spend the money on buying qi so that you meet the requirements to own the item. I don't know the exact price per qi, or who you can go to to buy it, but believe me when I say it's not cheap.
Also, by buying qi and such to get items that you don't meet the requirements for, you'll immediately earn the hate of a few members of the forum that see qi-buying as cheating, so don't expect everyone to love you because you've suddenly got a custom belt and are running around in elf and demon.