Hmm, My immediate thought actually is that there's a fucking huge overlap. Print your own cornicing for your house
In 30cm pieces? Taking 12 hours for enough to do a single room?
... Print your own bath plugs...
Yes, you can save $0.5 USD just by waiting an hour.
Print your own custom pipes for the awkward places that are unique to your house...
No such thing - a variety of elbows, t-pieces and such are available to fit any sort of plumbing the average house has, which leaves us to the only legitimate use ...
Print your own parts to customise fixtures and fittings.
And this is the option that requires the owner to be proficient enough to design his own stuff in the first place, which may take (from my experience) anything from 2 hours to twenty hours. If he is downloading a design off of the net, then it's not unique, is it?
The list is basically endless in the DIY landscape.
I do lots of DIY. Really lots. I've refloored my house with three different types of flooring (wood, carpets and tiles), rebuilt walls, built a pond, welded up 30m of palisade fencing. I've built tiny once-off tools for specialised purposes (especially when repairing newer models of cars). I've reroofed my garage, installed ceilings, rewired entire buildings. I've done the plumbing and cabling for much of my house. I've built cupboards, and constructed various sheet metal projects. I'm okay with electronics as well, having had a career in the embedded world.
And with all that experience of rolling my own, I can tell you one thing for certain - there is no decent ROI in any of the current consumer 3d printers (I've got unlimited access to one at work). You cannot print anything other than cosmetic items due to the lack of strength or rigidity. The pipes you think you will print? Maybe it will work as a straight length - a single bend is going to cause unpredictable rigidity. It will also still have to be milled/finished to fit into the other fittings. Or perhaps you want to print out enclosures for your rasberry pi/arduino projects? Rather than get an ugly plastic case with lines all over it, you can construct one from aluminium sheets in about ten minutes with a jigsaw and a $20 home-made brake.
The only worry home depot might have is that there might be too much overlap, and their sales of other things might drop!
I doubt it - you can't even print a spanner that will work more than once. Or anything that is supposed to look good.