The only difference between making a fountain pen and rollerball is the front section. Whether you make a Foutain pen front section or Rollerball Front section/ nosecone,
You can make them so they will interchange as well.
Just to add to this thread. Selling history 50%! Lilac one completed Wednesday, sold the following Saturday. I can never work out why one pen sells quickly but another goes a year later.
You see more fountain pens posted mainly bc that's what's in demand. I make rollerballs, ballpoints in twist or click, and dip pens. They all sell, but the biggest draw are the fountain pens.