'Duplicating your items! no bells needed! free duplicating for everyone!' reads one thread on the Animal Crossing GameFaqs message board.
In addition, as I mentioned, the duping bug has also caused once rare items to flood the market. 7-11 items, for example, are part of a promotional set that is available only in Japan, and previously fetched a high price in the American market. Gold items, created via a rare resource, used to be similarly difficult to obtain. All of these items can now be obtained with little effort.
Golden tools, the result of completing specific in-game achievements, should be the rarest items of all, but after the duping bug? It's no trouble at all to find a willing seller.