Learn Polish: Nie można kupić każdej książki.

This is due to the genitive of negation. The verb kupić usually takes a direct object in the accusative case (for example, można kupić każdą książkę). However, when the governing verb phrase is negated—in this case, nie można—the direct object of the attached infinitive automatically switches to the genitive case, which gives us każdej książki.

Why use the perfective verb kupić instead of the imperfective kupować in a general statement like this?

Even though this is a general truth, kupić (perfective) emphasizes the completion or successful result of the action. The sentence means it is practically impossible to successfully acquire every single book. If you used kupować (imperfective), the phrase nie można kupować would often be understood as a soft prohibition, meaning something closer to 'one shouldn't make a habit of buying every book.'

How does nie można differ from nie wolno in this context?

Nie można expresses that an action is practically or physically impossible (for instance, because you lack the money, time, or space to own every book). If you used nie wolno, it would mean the action is strictly forbidden by a rule or authority, as if there were a literal law against buying every book.

Is it possible to change the word order to Nie można każdej książki kupić?