In many US states, even a verbal contract is legally binding. Obviously this depends on many factors, the terms have to be reasonable and it has to be clearly communicated (and it's generally harder to prove a verbal contract in court). Text and email communications are the same way.

In your example, it likely wouldn't be considered a valid contract though. The terms have to be reasonable for both parties and there has to be a clear understanding that it is an agreement. Getting "ok" back is presumably sarcastic or not genuine.

They had previously conducted business via text message, the business deal had been discussed previously and when asked for confirmation that the deal was good via a picture of the actual contract the farmer responded with a thumbs up. The buyer then had to purchase the goods at the current market rate that was significantly higher which put him out of pocket.

So the guy suing the farmer could show that not only was this a normal way of them doing business, but he had effectively written acceptance of the deal (the emoji) and incurred a substantial monetary loss as a result of the agreement not being honored.

Contracts are a meeting of minds with consideration for some benefit. I don't see why that can't be accomplished verbally, on WhatsApp, or on 500 sheets of foolscap, tbh.