Err this reply comes late nepe ji. But according to "law" (which is something we all bind ourselves with), a binding contract occurs when there is offer, consideration, intention, acceptance, legality and capacity. You made the offer- to sell a book. I accepted the offer and communicated it in a manner u wanted me to- paypal. The consideration on your part was the good- and on my part was the cash. Was there an intention to create a binding contract? yes. And was it legal? of course- the medium paypal is a registered institution and as far as capacity is concerned, electronic business law states anybody who clicks the "i agree" or "ok" button accepts the contract and is deemed capabale of enforcing it also. Thus the moment I sent u my money, I owned the book as per our contract. And defaulting on our contract, the court would order you to hand me the book so that both parites (u and me) would be in our ideal state.