It's likely that other casual agreements that you don't think of as contracts are also contracts.
I'm not a lawyer, I'm a guy who took a business law class during undergrad, but my prof drilled it into my head that a contract consists of offer, acceptance, consideration, capacity, and legality. It can be on a napkin, it can be verbal, it can be in a text message, so long as those elements are present.
The difference between an offer and preliminary negotiations is the intent to contract, and language can be used to provide evidence on either side of this one. "Would you take $100k with a $5MM cap" is negotiating language, while "I'll give you $100k with a $5MM cap" is an offer.
Again, not a lawyer, but this "handshake protocol" appears to create valid contracts.