You're saying they should be responsible. Ethically? Legally? I'm not sure which you mean, but probably both.
That's not the question I was answering though. That question didn't specify the flavor of responsibility, and I chose to answer it from a mostly legal perspective, which is that as things stand they are probably, mostly, not liable.
Traditional retail liability is probably the best place to look in this case. A store can be liable for the products it sells, but if it makes reasonable efforts to determine product safety then those are difficult cases to win unless you can show that the retailer knew, or should have known, that the product was defective or unsafe. One black & white example of that liability would be selling alcohol to underage kids who did not present any ID, or gave a fake ID.
I think "reasonable precautions" is probably the best rule from a practical standpoint. But I'm not otherwise going to address where the line should be drawn on "reasonable" precautions. That's a complex question, individual examples and product classes would vary, and there are plenty of expensive court cases that have not yet produced a universal "bright line" standard for defining "reasonable" precautions.
And yes, I think MS should be responsible for apps they are selling. Aswell as Apple or Google.
Hiding behind adds and pretending it is no a proper sale and that they are mediators or whatever is BS.