title_body: Selling ebooks of school textbooks
upvoted_answer: This is not legal advice, but the answer to your question is almost always no.
  
  In general, it is illegal to sell a copy (physical or electronic) of a book if you are also keeping a copy. Say you own a physical copy of a book. You can sell that copy, but you generally cannot sell an electronic copy, regardless of whether you keep the physical copy. This is also generally true even if the buyer owns a physical or electronic copy of the book already. This is because copyright protects the copyright owner's exclusive right to make or distribute copies in the first place.
  
  If you are even thinking of doing this, you absolutely must consult a copyright lawyer. There are very large potential penalties for copyright infringement, regardless of the amount of money changing hands. For example, one person was ordered to pay $1.5 million for sharing 24 mp3s.
  
  So, the answer is: probably not. Tread with extreme care and consult a lawyer.