Two choices:

1. find a price that discourages the desire to sell a “used” e-book
2. spend a lot of time and money persecuting your customers before you find yourself with only 1 choice (see above)

Why does anyone want to sell something that takes up no room, can be deleted/thrown away with no consequence or remorse? Because they paid more than it is now worth to them (value approaching $0) though it still has value to next year’s student in that course. 

Looks like a rental market waiting to happen, as see with streaming movies — pay a fee, view the material within a set number of days, then it disappears.