Your ring looks nice but the only reasonable choice when printing it is silver (who would want a plastic one) plus you have to have your diamonds and fit them yourself, not everyone know how to do this, where to get them. Can not tell you much more about 3D printing as I have no experience in that field.
Your stool and knife are sold as 3D models but only have one file format. Most popular formats for 3d models are .obj and .max. It is always good to export your model to the most popular formats (others are .fbx, .3ds). Presentation - need more preview images, wire-frame and clay renders are advisable as they show customer the topology. Also, search for similar models on the market, compare them to yours, see what aspects you can improve to better compete with others.
