Implicit contracts are not valid. this is the core of libertarian philosophy, and why, if they're self-honest, libertarians are against IP.
So if I walk into a candy store, put a candy bar and a dollar on the counter, what happens? Can the candy store owner just take my dollar and not give me the candy bar? How do I know the candy bar is mine after I've paid?
I have met dozens of Libertarians and I've never met a single one who believed that implied contracts were invalid where the terms were not unconscionable, known to both parties, and one party gave the consideration to the other that was understood by both to signify acceptance of the contract.
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.