This doesn't directly address your question, BlaisePascal, but I seem to recall an O'Reilly policy that discounts purchase price if you already own a prior edition of one of their books.   If I can find details, I post them here in an Update.

Update: Found it: www.oreilly.com/order/upgrade.html   Provides 30% discount with original title page of older edition.   Dunno how that compares to prices at fatbrain
    cheers,
    Don   (ybiC)
    striving for Perl Adept