There is no such thing as the "sharing economy". Either you have a real business or you are doing piece work (Uber is a "piece work" economy )
AutoCAD has some tools for doing this, a new startup is specifically focused in doing 3D design in the cloud, called OnShape.com
But if you are talking about a manufacturable component, there needs to be quite a bit of precision in the design. Sure you can 3D print some of it - but 3D printing needs quite a bit of post processing and is limited in the materials you can use.
Autodesk and some other companies have done "plug and play" design tools for interior design and some companies have done "assemble the legos" style design
but to be able to allow someone to design say the details of a table leg and a table is quite hard. Because you have to have a very precise result for manufacturability and you also need to have strength in the right places etc.
And how this would work in a "sharing economy" really doesn't make a lot of sense. Because what you are talking about is that someone would take the time to assemble "build to order" level of specifications that then would get bid out to a marketplace of craftspeople for implementation
Think of the challenges involved
Essentially you are talking about the Amazon Sales Partner model, but where instead of the Sales Partners offering a service and customers choosing whether or not to buy, you are adding the complexity of the design process between them.
Good luck... you will need it