Personally, I am interested in any U.S. style Railroad Steam Locomotive models (well, real ones too!). But that is not a common item liked by the masses.
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When I make 3-D prints they are usually just something I want, and not something that is worth wide distribution/sales, so I don't offer them for sale anywhere.
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From my viewpoint, if you want something that is not commercially available at a price you find palatable, design the item yourself and make it for yourself. If it happens to be something that others might be interested in, then open a shop on ShapeWays and see if you can direct traffic to your shop via ads on Facebook, or other social media, or your personal blog or whatever you can think of.
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To see the wide range of things already offered, click "MarketPlace" above and wander through the various shops. I think you will find jewelry is the most common items, and the rest is probably being sold because the designer/shop owner wanted the item for themselves and put it in their shop on the off chance that someone else might want one too, but they don't get much in the way of sales.