What does 到 mean in 哪里可以买到正版的这本书? Does 买到 here serve as a verb-object word?
「到」(literally to arrive) is a verb complement to show whether an action has or can be completed (“arrived at the finished state”).

Since「到」is not actually necessary in this sentence to express the same meaning, it is easier to demonstrate the meaning of「到」with a complementary sentence:


  那裏買不到正版的這本書
  
  The proper version of this book can’t be bought there.


X「到」means the action X can be completed, X「不到」means it cannot be completed.