1) The Beit Meir, at beginning of Shulchan Aruch Orach Chaim #369, writes that he does not know what forced Rashi into writing that the shop owner or baker live in the same mavoy or chatzer as the person giving him the money.

2) However, we may be able to understand this if we think about the way the Tiferet Yisrael #45 explains; that the neighbor says to the shopkeeper or baker that when the other residents of the chatzer or mavoy come to buy from you bread for the eruv, please give them a little more bread for me.

3) What is happening is that the residents have decided that they need to make an eruv of the chatzer or a shituf of the mavoy, and to do this one needs bread or wine. The obvious person to do this for them is the local shop owner, because he has bread or wine. Therefore the neighbors go to the shop owner and give him money to purchase their share of the eruv. Our Mishnah is discussing one of the residents who went on his own to the shopkeeper, and anticipates that at some later stage his neighbors will also approach him in order to make an eruv or shituf together with him. The first neighbor gets in first and asks the shopkeeper to give a bit more food so that will be for his part of the eruv.

4) If there would not be a shop owner who also lived in the mavoy or chatzer the residents would go and buy bread somewhere else and put it in the house of one of the residents. But since it so happens that there is a shop owner who also lives here, his house is the obvious address to put the eruv in. This is why Rashi writes that the baker is "dar imo"; since it now follows that the eruv can be placed in his house.

[see sefer Or Tzvi]

Dovid Bloom