Well, none of us can really say what your seller did or did not do to help this situation along because we were not the ones receiving the plant. But, with root rot THAT severe in only 7 days, I have to agree, the soil mix was likely a contributing factor.
I do not see evidence of this being a stem cutting either from what I can see in the photos, but, it could be a side shoot propagation. I do not take stem top cuttings off of any of my variegated Monsteras. I only propagate side shoots that come off the parent plants, root themselves into the ground and start growing. But then the leaves on my parents are over 2 feet across and over 2 ft long and the stems are the diameter of the average flashlight handle.
I agree that I would have unpotted the plant pretty much immediately to check things out. And I would have repotted it into my own aroid mix that I make myself, unless I was planting it into the ground.
I would show the seller the photos and not be shy about asking about a partial refund, and would cite the other negative reviews. All they can do is say NO after all.
Meanwhile what I would do with the 'cutting' is remove that yellowed leaf. It is toast and will not recover. The plant will have a hard enough time trying to support the other leaves now that it has no root system.
I would treat the entire lower portion where the nodes and roots start with a good fungicide, a real fungicide, not cinnamon, and I would dip it into rooting hormone and place it back in the sphagnum. Unless it is really cool or cold where you are I would lose the plastic bag. Room temperature is fine. Just keep the moss damp, not wet, with a spray bottle and mist the leaves occasionally. Don;t be surprised if you lose more leaves. Keep it in a bright light position.
Part of what contributed to the failure of the root system from what I can see in your photos is that they planted the rooted cutting still in the spaghnum moss into a soil that was too water retentive. All the water is going to gravitate to that moss ball, which looks sizable, and just sit there. Its okay to place a plant with its moss ball into a medium that is porous light and fast draining, but it should be demossed first down to the minimum that can be removed without actually touching the roots with your fingers or pulling at them to get the moss off.
I have propagated Monsteras many times, its not hard to do. I never try to root in water, I usually just put them straight into my soil mix or plant them in the ground. I have never lost one yet. But then I really am a grower, I don't propagate just to sell stuff. And I do not import anything for resale. That takes a special addition to a nursery license, and most of the people selling stuff on etsy, eBay and Facebook don;t even have a nursery license, much less the import stamp
