Flash carts allow you to download new games/use save states, repros aren't flash carts, they're homebrew versions of the original game. Also unless they alter the rom in some way, it's pretty much impossible to tell if someone is playing on a repro cart unless they pull it out of the console and show the camera, so I don't see why repros are a no-go

Does using the built in save state on the practice rom damage the Nes Classic in any way? Sorry if this is a noob question, but i'm paranoid.