Here is the item again:
Darryl has the messiest book bag
(A) of anyone
Rachel knows;
(B) he essentially
has a compost pile rotting in the main
compartment. (C)
For instance,
pistachio shells, pencil shavings, and a brown banana.
You correctly realized that the phrase following for instance is a an afterthought fragment. You chose to revise the fragment so that it became a complete sentence, like this:
Darryl has the messiest book bag of anyone Rachel knows;
he essentially has a compost pile rotting in the main
compartment. For instance,
there are pistachio shells, pencil shavings, and a brown banana.
Nicely done!