Selling print online is very difficult as more and more printers move online, trust me the "build a website and they will come" saying is faaaaaaaaaaaaaaaaaaaaaaaaar from the truth, personally to start off I would concentrate on your local market not online, try and win business from the customers still paying top price at Prontaprint etc, give them the same quality, service and save them some money and you could do well.
From our own print reseller scheme the people that do well are graphic designers as they don't have to rely just on the print income, it is more of a bolt on to their existing business. To start off and test the water a lot of them just created a section on their exisiting design website called print and listed some popular products, quantities and prices.