Damn, I've just looked into this at Sythe.
And it is most likely duped money.. Considering this is going about 200B+ right now I think that Jagex might actually do something. This guy also knows the dupe, pretty good friend of mine and 700+ Vouches at Sythe.
http://www.sythe.org/showthread.php?p=11292412#post11292412All bulk sellers are buying off him and selling for ~0.45 which is quite a nice profit margin. But they are all purchasing duped gold and re-selling it to their customers which means Jagex could easily backtrack these people if they wanted. Jagex has deleted banks of certain big RWT'ers in the past.
I would be careful with buying from those people as it is almost certainly duped money.
Just.. knowing that Jagex are money hungry I don't think they would let someone make 100.000$ in a few days off their game.