Keefe the Thief
Hero
Except for those that need a 'dead tree version' you don't ever have to buy the Core books for PF, they're free online at Paizo, d20pfsrd.com and at a number of other sites - the PRD is complete with everything (as everything in PF is OGC.) Even the Bestiary and APG new classes are online as well. Now some of the Golarian stuff isn't necessarily OGC, so that and the APs is all you'll be spending money on - if you need the setting. As long as you have an active internet connection while you're playing tabletop, you can always refer to the online source. Better yet, for 9.99 you can buy full PDFs of every book that they've produced so far.
Either way, no reason to buy a whole new set of books - this applies Umbran's point as well.
GP
I could be wrong but i think those who said they didn´t want to buy new books actually want to buy and use books when playing D&D. So, if they want to play Pathfinder, they need to buy books.