When I sent an iMac to Apple's trade-in/recycling service (that is operated by a non-Apple company), the box, padding, and instruction sheet I was required to use looked like the items in your photos. So it is possible the seller of the iMac is trying to see if you will pay them more than Apple has offered them for a trade-in before dispatching it. Plus they will save money on a private sale because Apple has provided them with a custom shipping box.
Further, when Apple packs desktop computers to send to buyers, including official Apple refurbs, the outer and inner packaging is a lot different.
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ETA: I don't feel trade-in/recycling packaging is as protective as purchased-item packaging because there are fewer foam inserts, no screen protection cover, and no provision for holding in a keyboard and mouse securely. These differences aren't that important for stuff that is going to be recycled or stripped for parts, of course, but may be insufficient for a computer that is going to continue being used.
Arrived, and all is fine! Like in the original Apple box, the iMac's stand does not touch the bottom of the box, i.e., the styrofoam fully supports the machine. I would still consider the serial box a touch more secure though as the clearance there is more pronounced.