Here's something that I wished existed. After getting a 3D printer, and learning modeling software, I'm constantly creating "stuff" that doesn't exist in any store, but satisfy needs I've had around the house. Now there are 3D model hosting sites where you can sell downloads of your model file. And there are 3D printing farms where you can have them crank out and ship you any quantity of your models. I want that combined into one B2B entity where I can place a listing for one of my products on something like Ebay or Amazon. Then when a customer orders it, have it fulfilled by the 3D print farm company, package it up with my product instructions, and ship it out to the customer (and once volume hits a certain level, maintain inventory at Amazon for example). They collect payment and send me the difference between my sell price and their manufacturing price, with automatic price breaks when volume hits a specific level.
Objects that I've designed and 3D printed for myself include things like Dremel tool attachments, dust port attachments for my router table, nut/washer holders that you screw on the bottom of a table to hold the nut in place (so you can move a top post from one location to another as needed without reaching under the table to hold onto the nut), things like that. Some small items, some larger mechanical devices.
The closest I've seen to this is some woodworkers on Youtube run their own store and have a small number of 3D printers for cranking out the products they sell. I want something like this scaled up for any designer.