advanced potion-making

[klodiana_xha]

No: HPFGUIDX 137960

I don't know if you noticed this but when Harry tries to find out if
his dad is the HBP, Lupin tells him to check out how old the book was.
The book was 50 years old, so Harry dismisses the possibility that it
was James' or any other's who was around at his time. 
How come Snape is the owner of the book? Is it because he uses
secondhand books (do we know that?)? Is it just an error or something else?

Klodiana