Nope. You are incorrect. Retailers do NOT purchase these. They remain Nintendo's property the entire time.
Please stop twisting my words to fit your story. I did not say Ninty was definitely, positively, 100% going to come after you, and I never commented on the console's lifecycle. I said they can come after you to take back their property, and that the more you advertise that you have it, the more likely they are to want it back. Nintendo cares very much about their property. Even demo carts are their property and are supposed to be sent back after use.
You asked how to get more demos. I told you the two methods of obtaining demos. I also told you that demos are not tied to any specific demo unit and can be used on any demo unit, so if you happen to find any of the discs, you would be able to use them. The only issue that stems from that is if the disc has an older system firmware, then I would doubt it would install the demo from it, as it is a system update disc, not a demo update disc.
That pretty much should clear the air for you as to how to obtain demos. As you should already know, retail games will not work on demo units, so you can't go that route. The above methods are the only ways. So I am unsure as to how you don't think I answered your question. I also was kind enough to provide you with information regarding who owns your device: Nintendo, not you. And again, no, Nintendo is not guaranteed to reclaim their property back from you, but they can, and you should be aware of that.
Lastly, the company/person that sold you this unit broke the law, and sold you something they do not own. Do what you want with that knowledge.