[alfonsodev]

Might be a case of money laundry ? You would think it would be one-off then, but adding more units would make it more believable.

Example: it is very common to find people selling a pen for 50 Euro, by buying the pen you are gifted with concert tickets which are illegal to re-sell.

So think anything ilegal to sell or serve, that can be gifted if you buy that book.

edit: I agree with what user WaitWaitWha is saying.