In real world (as opposed to theoretical libertarian utopia), voluntary market interactions aren't that common. People are trivially manipulated.

Consider that the entire marketing industry is literally about taking away agency from one side of the business relationship.

Manipulation != Coercion.

Yes both are problems but libertarianism claims to solve only one of those. For the other one, if you are unable, follow/place-your-trust-on someone else. There is no need to solve one problem by creating bigger problem.

Consent is not the same as informed consent. Libertarian utopia is ripe for getting people tricked and robbed while technically fulfilling contracts to the letter.

Consider what would happen in, say, medicine without the informed part.

And saying you can switch... Typically not due to availability, lack of information and monopolies.

The other question is who enforces these contracts and who prevents these people from abusing their means.