The MakerBot 3-D printer offers the possibility of manufacturing products in the home almost as easily as printing a document with an inkjet printer. The company’s Thing-O-Matic machine moves in three dimensions to spray layers of colored plastic to form objects, some quite intricate. Do-it-yourselfers are already selling 3-D printed products like custom dollhouse furniture at art fairs and museum shops and on Web sites like Etsy.com . The price is $1,300 for a kit you put together yourself; a fully assembled machine costs $2,500. While this machine uses plastic, other 3-D printers can create objects made of metals and other materials. (The New York Times Company is an investor in True Ventures, one of a number of venture capital firms that has invested in MakerBot Industries.)