Great question!
minnowbooster is offering a great service as well. We would say that we are both equal from the profitability point of view. The differences for smartsteem (smartmarket) are, that you are able to select specific groups of users to make sure that you're selling your vote to quality content.
Here is the list: https://smartsteem.com/users/whitelist
So by using our service - you would also support our development in this area, since we are only 2 people behind smartsteem / smartmarket - so a bit smaller than minnowbooster :)
But you can of course always enable both services - if you like. That way you would make sure that your votes always gets sold.
Good to hear, there is a whole market for voteselling(especially when sbd keeps up its price :) there will be plenty of votesellers :) anyway, i believe there is room to grow.