I never said verbal agreements were not valid. Or even the short-hand agreements that take the form of pricetag. You see the price tag and you know, that if you give the clerk that specified amount of money, you can leave the store with the item. That is an explicit, if abbreviated, agreement between you and the store.
There is one and only one difference between an implied contract and an explicit contract. With an explicit contract, the terms are explicitly agreed to. With an implicit contract, the contract is offered by one side and the agreement is implied from the other side's performance of what the implicit contract requires.
For example: "How much is a candy bar?" "$1" "Okay" - explicit
"How much is a candy bar?" "$1" [hands $1 across the counter] - implicit
That really is the only difference. In both cases, some of the terms of the contract are implied by law and understood by both sides. (For example, what happens if my dollar is counterfeit? What happens if the candy bar is defective in some way? All of these terms are implicitly part of even an explicit contract.)
There really is almost no difference. Most real-world agreements have both explicit and implicit terms. The only real difference is that it's sometimes harder to prove that an implicit contract exists or to establish that it had particular terms. But so long as there was a mutual understanding, whether explicit or implicit, the terms should be enforceable. (With the obvious exceptions for things like verbal agreements to transfer ownership in land, unconscionable terms, agreements obtained by coercion, and so on.)