After trying most known services for selling your votes:
I found that Smartsteem is the most rewarding vote selling service if you are not a regular writer In steemit but if you are a regular writer ( 3 posts in the week at least)
the best service for you is Shadowbot.
I did not found MinnowBooster very rewarding like the other two services.
If someone found other results please inform us in a comment in this post.
And have a nice day.