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!