I believe parent comment is referring to "consideration" in the legal context commonly used in contracts to distinguish between unilateral and bilateral agreements.

https://legal-dictionary.thefreedictionary.com/consideration

https://en.wikipedia.org/wiki/Consideration

Not a lawyer, but I believe that just giving something away is not technically enforceable which is why you see 1 dollar price tags, and picking it up would count as payment if the person giving it away is trying to get rid of it, the value is kind of in the garbage disposal aspect. And its not that you cant list or pick up stuff for free, you bn obviously can, but you just might not be forming a legally enforceable contract