All it means is you have to follow the instructions to set it up; it doesn't come working out of the box. Yes, it's legal to sell products that require setup and are only partially assembled out of the box. Think of a more extreme example like a backyard swing set for kids that comes all packed up in a box. The keyboard is guaranteed to work after you follow the setup, and you can send an email for help if you are stuck.

Probably, if "for parts or not working" is a valid description for items on eBay.