But why buy votes, if one could buy rather Hive instead and don´t have to give a part of the money to the voting service provider? It is like buying your own home over time (=building up HP) is better than paying rent for the rest of your life.
On ecency there is a system where you can collect points just by being active and then use those to promote your posts. Better than buying votes.
And filtering for quality never has worked. Although can´t rule out that a new creative idea may bring the solution.