If you have that machine, and the means to power
My 3D printer uses about 20 watts. So the power costs about a penny per day.
Does that power come from a power grid, or from solar? Who makes the semiconductors for the solar panels, and the chips in the printer? How much did they cost and how did you pay for them? How long does it take you to make an item like, say, a bucket?
The robots should be able to repair and maintain each other. If not, then that is job for someone!
If the robots aren't up to repairing each other, and unless that someone is me, I will need to pay them. What should I pay them with - 3D printed goods that they can make themselves?
and keep it fed with the necessary materials
Current replicators use extruded plastic, but people are already working on making them work with shredded recycled plastic, and recycled powdered metal. So if you run out of raw materials, just go gather up some bottles or cans from the side of the road.
Just as the world's bio-diesel needs can not be met from recycled takeaway fryer oil, the "pick up other people's discarded stuff to feed my 3D printer" model is just as unscalable. Scattered, cottage industry is not the same as keeping the world running using only waste stuff.
Look, I think that 3D printing is nifty, and recycling stuff is awesome, and using things that people don't want is great - I'm one of those people that hardly ever buys anything technological until it goes on closeout sale - but it's a big stretch to claim that owning a magic printing machine and an R2-D2 to fix it will allow people to survive in a future where their labour has little selling proposition, let alone a unique selling proposition. What I want to understand is how owning some robot buddies lets you either live decently while sitting completely apart from the post-singularity economy, or participate meaningfully in it, when everything that you can do could be done by anyone else with the same gadgets.