There's a few options ranging from subscription to fully built custom web apps. It's going to range from a couple hundred a month to thousands a month or tens of thousands to develop. I can't remember the names, but there is one for Magento and Word Press. There's options from EFI and HP if you have their machines. There are some companies that do fulfillment like 4over and ZooPrinting that have templates setup for small format options, but no banners or signs yet.
The main reason you don't see this stuff more, is because it doesn't work. Customers can't look at a blank canvas and create something that looks good. If they can, they likely already have software for it. Unless you're ready to create hundreds of templates in multiple sizes and have hundreds of stock images and art options ready, as well as being willing to teach customers how to use it every day, then by all means, give it a shot.
These types of systems are only successful when they have templates and predefined text inputs. i.e. Business Cards for a franchise that all they do is put in their contact info and submit the order on a predefined and approved layout. Or Vista Print where they have matching templates so you can get everything from business cards to yard signs that all look the same.
When one of these sales guy selling an online tool tells you that tons of people use it. It's typically companies that tried it and stopped, people that are in the process of trying it, or companies that use it for their B2B stuff with preapproved layouts.
