I don't think you represent the average. Also, publishers try to deter this sort of activity by publishing updated editions every 2-4 years. This messes you up if you get holding the ball or if this year is the first edition. It also discourages booksellers from holding used books in stock (you can't send those back).

Then you have people keeping textbooks for various reasons. One of them is that it's hard to buy/sell books. You need to get in early.

Your $150 relies on both buying & selling used.