That's not really correct - you just have to promise to exchange something. If you and I shake on a deal to buy your car for $100, that can certainly be legally

That's not really correct - you just have to promise to exchange something. If you and I shake on a deal to buy your car for $100, that can certainly be legally binding if we intended it to be so. Usually, though, handshake deals mean "these are the commercial terms, let's go draw up the contract", which isn't legally binding. In many places, though, they're respected by convention and protected by reputation.