I'm wondering if things like LEGO store plastic bags, LEGO boxes, instruction booklets, packaging etc., count as "LEGO pieces" if I incorporate them into a build for the purposes of posting as a purist LEGO MOC?
There is no official answer on this, as “purist” LEGO is a consensus, not a definition. That said, there are several examples of pieces LEGO doesn’t classify as a part that are used in MOCs, such as:
- The pin used to keep bundles of 4 train tracks together.
- This technic hub piece was never intended for standalone use (it was part of a pin joiner) but has been used by MOCcers due to having antistuds on both sides (within a 1x1 brick footprint).
- The iconic brick separator is not intended as a regular LEGO piece, but has a whole class of MOCs based on it. I have seen it used as railway station roofs, spaceship wings etc.
Net, I think the general consensus is that if it has been produced by LEGO and you can find a way to incorporate it into a MOC, your creation is still considered “purist”. At the least it gives you good arguments for any debate that might arise.
Curious to see how you will incorporate shopping bags in a MOC ... I can imagine one being used in a rather large scale MOC where you utilize the LEGO logo on it as part of a display.
Another Lego byproduct that gets reused from time to time are "sprues". Some elements are moulded by having channels directing the plastic to the various part of the element. The remnant plastic in these channels is called the sprue (confusingly the channels themselves are called the sprue as well) Some Lego elements are provided with a sprue still attached.
Two examples come to mind: The sprue of this plant element
is often used as vines
And the magic wand elements from the Harry Potter series also come with a sprue, here is an example of them used as a decorative element in a wall
More uses of the Harry Potter magic wand sprue can be found here: http://www.newelementary.com/2018/08/how-to-use-lego-harry-potter-wand-sprue.html