Exactly. Well said!

If I'm looking at the correct book, it was published in 1999. I would assume resales would increase in some sort of exponential fashion over 10 years.

I doubt it's pirates. Your decline in sales is most likely due to students re-selling your used textbooks. Change a word (or two) and release a new edition to put a damper on used textbook sales. Students will love that. Probably so much that they would actually start to pirate your book...