I will note you used the term "transaction" rather than "interaction." A "transaction" is of course not something that exists in the state of nature. What exists are interactions. E.g. one animal killing and eating another animal. But a "transaction" is an interaction with the additional quality that it is defined relative to some legal framework. A "transaction" has no meaning without reference to a legal framework or the collective threat of force underlying that legal framework. And that legal framework is, of course, a completely arbitrary social construction.

Thus, it is not the transaction that creates the moral obligation. It is the imposition of the legal framework without which transactions would be merely interactions. That legal framework requires everyone to give up the only thing that can be called a "natural right"--the right to kill and eat whatever cross's one's path. The whole collectively sacrifices it's natural liberty in order to construct a structured society that enables wealth creation. It is that collective sacrifice that creates the collective moral obligation.

I'll give you a very concrete example. We make "conversion" a crime. That is basically using property for your own benefit that is in your possession by the voluntary action of another, but does not "belong" to you. Of course, "ownership" in any sense beyond physical possession is the product of a set of legal contrivances. Activity like embezzlement, etc, didn't actually used to be illegal until relatively recently. "Theft" was defined narrowly to taking from someone's physical possession. But we can't really have a modern economy without lower-level people taking physical possession of property that "belongs" to someone else. The truck driver, the parking garage attendant, the cashier, etc. So we alter the legal system to prohibit this certain activity presumably to increase the output of the economic engine. So when you leave your car with a parking garage attendant, you enter into a "transaction" for him to take care of your car, but that "transaction" is obviously meaningless without reference to the legal framework that's in place. And that legal framework involves people giving up the ability to do things they used to be able to do. And that creates the moral obligation.

Approaching the situation from another angle: your reverence for the "transaction" blinds you to the fact that they are nothing more than arbitrary accounting mechanisms within our arbitrary legal framework. At the physical level, the output of the economic engine of a city is the product of everyone's effort. Like a real engine, nothing goes if the fuel pump doesn't pump the gas, the cylinders don't compress the gas, the spark plug doesn't ignite the gas, the valves don't flush out waste products, etc. The fuel pump may consider itself the most important contributor, because after all there is one fuel pump and perhaps half a dozen spark plugs and a dozen or more valves, but the engine does zero work without all its elements. To the extent that anyone benefits from the output of the economic engine of a modern industrial city, they have a moral obligation to all the other elements without which that output wouldn't be possible. The fact that you use accounting mechanisms you call "transactions" to divvy up the output doesn't mean that those "transactions" are the things with real meaning at the physical level.