What is the Etiquette and Legality Around Selling 3D Prints Which I Didn't Design?

Greetings all, new to the hobby. I've been printing about a month, and have already had double digit successful prints of pretty darn good quality.

My kid wants to sell printed articulated dragons to their school friends. I'm not really looking at trying to sell at scale or anything, more just a few here and there to pay for purchasing new filament.

Do I have to learn modeling and design my own dragons to print in order to not be a complete a-hole? I feel like I see businesses selling the designs I've saved, so it makes me curious if I would just be doing a ton of extra work for nothing.

Thanks for giving a newbie your thoughts, I appreciate it!