Of course you can sell a game. But can you sell game rules? Can you sell rules and still make a buck?
No. Rule’s can’t be copyrighted and aren’t worth patenting (and a rules patent would probably be overturned in court if it came to that). Rules, in and of themselves, have no monetary value.
You could collect a large amount of rules and sell them as a book. If the rules referred to games you could play using standard components. If there were enough of them. If they were of games similar to what people were already playing, or aimed at a specialist audience. And you probably wouldn’t make much with it. So no, even if you could sell rules you wouldn’t make much money selling them.