Yeap, Steemit is a free market, therefore the community decides what behaviors to promote and what behaviors to restrain. Users who invested the most in the platform and have the biggest stake in the game (most steem / steep power) have interest in the platform performing good for the community, so their stake is worth something, therefore if the community feels that buying votes have negative impact on the platform, these high stake individuals will probably act against it and they have tools to do so (eg flag, affect user's reputation). At the other hand, if users find buying votes as a genuine part of the game and positive then people buying / selling votes have nothing to worry about. It's all about perception and platform advancement.