yeah, it's a hard line to walk, but vote buying is clearly advertising which should put the post in the promotion tab. The trending tab is for posts found valuable by the community, which posts that make it there by being voted by voting bots clearly are not. Posts may be both advertising and found to be valuable, but there is probably a metric that can be analyzed to determine if it is one and not the other. I guess at the dollar mark that causes the post to be on the trending page maybe determine the percentage of author reward that can be attributed to voting bots and consider it as not community supported if the majority is voting bots. Remove your post if the tide swings the other way. This type of thing needs some tweaking, but aside from a major upheaval like SMT proof of brain authentication being the only way for a post to get on the trending page, I'm not sure what else there is. Just an idea...