I disagree. Verbal contracts without proper consideration is non-binding.
The consideration is a promise to perform (i.e. to pay) in the future.
If you were to create a written contract it might read "In consideration of Mr. Beluga's promise to purchase XYZ knifer on or before Friday, February 28, 2003 for the sum of $100, Acme Cutlery company agrees not to sell said knife to another customer before noon on the aforementioned date."
Is there value in Mr. Beluga's consideration, his promise to purchase the knife? Of course. There is value for Acme Cutlery in knowing that they have a reasonable expectation of selling that knife on a certain date. That knife could languish in their inventory for weeks, months, years until an interested buyer arrives. Acme's got an interested buyer right now. He just needs a few days to get his money straightened out. But, what if, in the mean time, Mr. Beluga sees another knife he likes even more? Maybe he'll change his mind and buy it instead and Acme will be, once again, stuck with this knife that could take a long time to sell. To, to assure that this doesn't happen, to bind Mr. Beluga to purchase the knife, something which is valuable to Acme, Acme enters into a contract with Mr. Beluga. They get what they want, a guarantee that in the very near future this knife will be sold and out of their inventory even if another prospective buyer doesn't come along. And Mr. Mr. Beluga gets what he wants, a guarantee that they won't sell the knife within the next few days if another buyer does come along.
You see, there's risk on both sides. For Acme, the risk is that Mr. Beluga will change his mind and not purchase the knife and that, after he does this, no other buyer will come along for some potentially infinite amount of time. They risk having to carry this knife on their inventory forever. The fact that they were willing to enter into such a contract indicates that they perceive this risk.
Mr. Beluga, on the other hand, has just the opposite risk. For Mr. Beluga, the risk is that another buyer will come along and buy the knife before he's able to get his finances straighten out and that Acme will sell the knife to that other buyer. The fact that he asked for this contract indicates that he perceives this risk.
Both parties perceive risk, albeit different risks, but risks nonetheless. The contract mitigates the risk for both parties. Mitigating risk is generally seen as valuable. Therefore, mitigating risk can be a consideration in and of itself.
Mr. Beluga's promise of future performance, his promise to purchase the knife mitigated risk for Acme Cutlery. Mr. Beluga didn't have to do that. Therefore, that promise is valuable to Acme. Acme's promise to hold the knife for him mitigated risk for Mr. Beluga. Therefore, that promise is valuable to him. Both receive something of value. Is the value equal? It's hard to put a price on risk mitigation. The two parties seemed to, at the time, think it was a fair agreement. Both were willing to make it. If two parties, albeit bias parties, feel the value is equal, that's at least a sign that it is.
This type of contract (though usually written) is especially common in real estate and also sometimes seen in automobiles. A person sees a house they want. But, this potential buyer needs a few days to organize a mortgage. The potential buyer is afraid that the seller will sell the house to someone else before he can get that paperwork processed. The seller believes that the potential buyer will be able to secure financing. But, he's afraid that the potential buyer will change his mind before the loan is approved. So, they enter into the exact same agreement.
It doesn't even have to be so balanced. When I bought my first house, I was all pre-approved for my mortage and everything. I liked the place, but I wanted to think about it for a bit, "sleep on it," and get a couple of second opinions from some trusted friends. So, the seller and I entered into a written agreement in which they held the property for me for 48 hours basically in consideration of my promise to think about it. In that case, even my simple act of "thinking about it," has legal value. It's valuable to the seller to have a potential buyer seriously consider the house.
The problem we have in Mr. Beluga's case is that it's a verbal contract probably without any witnesses. So, it'd be very hard to enforce legally.
But, this is where what is legal and what is ethical diverge. The informal, verbal nature of the agreement may make it legally possible for Acme to break their agreement. But it's still not ethical for them to do so.
Sometimes, there is something higher and more important than the letter of the law. Sometimes (for some people, always), ethics, doing the right thing, is more important.