Well, the home 3D printers use a completely different technology than the SLS 3D printers that shapeways uses. Many of the designs would not even print correctly on a home printer. Also, shapeways can physically ensure a print is correct before they ship it to a customer, which helps reduce refunds and unhappy customers, but if they sold 3D designs, how could they even be sure that the customer has a home printer that will successfully print that object? Would they want to deal with the possible negative feedback/reputation that might come with customers who fail to print a design they bought, even if the fault was with that person's home printer/settings?