RE: Don't use vote-selling bots: use @promoted instead. A bot that upvotes you when you burn money!
Easier solution is to move promoted posts into the normal feeds of /trending ect. No one takes "promoted" seriously because it is paid advertising. Why would you look there? You wouldn't. Advertisers pay to have content displayed along with normal content, just like in Google search. This is the model that has been proven to work online, it's how "promoted" should function. You just run it like Adwords where the top couple spots get fed in promoted posts using an algorithm to cycle the right ones into it based on what people are paying.