Answer:
At the beginning of the semester in gained $200 in value.
At the end of the semester I gained $75.
At the beginning of the semester, I actually paid $250 per book while i could have paid $450 for the book.
The difference between these values represent the consumer surplus.
Consumer surplus is defined as the difference between what a consumer is willing to pay and what s/he actually pays for a product or service.
At the beginning of the semester I had a consumer surplus of 
At the end of the semester, the book is worth only $50 to me, but the book store will pay $125 for the same book.
So, I gain
by selling the book to the bookstore at the end of the year.