"... But a great business idea would a small shop, that owns one of these and sells its use. You send them your plans, come in the next day and pick up the part. ..."
If you have taken a classes in polymer chemistry you will have fun choosing the materials, the properties of those materials & associated costs when you choose to construct your "thing" ~ http://www.shapeways.com/about/material-options
You mean something like this ~ http://www.shapeways.com Shapeways allows you to make some pretty interesting 3D things. You can read more about them in their blog ~ http://www.shapeways.com/blog/
If you have taken a classes in polymer chemistry you will have fun choosing the materials, the properties of those materials & associated costs when you choose to construct your "thing" ~ http://www.shapeways.com/about/material-options