A bestseller is a good selling book
A bestseller is actually a translation from English: Best sold. It is a name for a book that sells exceptionally well within a certain period (often in the first period of edition). It's hard to say what the numbers are. A Harry Potter or Dan Brown books sells more than a poet bundle. But this poet bundle can also be used as a bestselling book. It also happens that a book has been a long time old, but that certain events cause people to suddenly get interested in the book and thus still have a bestseller born.