This is already a possibility even without flash carts. It doesn't happen.
People dumping and sharing games know how it works, and have no desire to poison the used market. All that would do is turn the general public against everything/everyone leading up to such a device being possible.
The average person is not going to buy physical games, dump them (it's not backup if you don't keep it), and then resell them. You get too little of a return from reselling; if your goal is to have a game with as little money as possible out of your pocket, and you're willing to do something illegal (keeping a "backup" of a game that you resell)... you'd just download somebody else's dump. Which you can do for the vast majority of Switch games the day they're released if not earlier.
And that's aside from the fact that the person buying, dumping, and reselling a game would get banned too. As well as losing the difference of what they bought it for vs what they could resell it for, and having the person they sold it to asking for restitution.
So, there's just no reason to buy, dump, and resell. It's all downside and no benefit.
Also, the majority of people dumping games want to keep the physical game. They're genuinely creating backups, or they're collectors and/or preservationists, or - more rarely - they somehow got a copy early and want to keep it for sentimental reasons (or, if they share it, are afraid it will be traced back to them). They're not evil pirates out to hurt used game buyers for the sake of being evil.