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.
This is wrong.
A valid contract requires consideration. That's one of the conditions I was referring to. A handshake and a deposit, for instance, could result in losing a lot more than just the deposit. However, not all consideration is monetary, which is why it helps to consult an expert.
I am not a lawyer, either. Just an engineer with the minimum legal training required for licensure, which is basically, "know when you should ask for help."
This is right, but misleading. Consideration isn't the big question in this thread. It's whether or not a verbal agreement can be binding.
In some cases it can be, but a mere agreement to agree is not binding.
The biggest takehome is 'consult with legal before making promises or representations to counterparties".
Edited this a few times to keep it as short as possible without being misleading.
At least, that's what I (mis-)remembered from a Contract Law 101 course taken long ago during my undergrad days.