Deal Breaker, Harlan Coben

As I intimated four days ago after finishing Harlan Coben's Long Lost I went off and did some investigations into what sequence the books should be read and whether we had any. Initially I thought I'd read this one but reading snippets they didn't seem familiar, so I read it and I hadn't and it was meant to be the first in the sequence. That's two books I've read in five days - unheard of - and after my investigations there's a few more on order. How come I can buy them online for less than we bought this one in a charity shop?