Since buying votes is much like flagging/downvoting content producers who do not buy votes, we can already see a world where quality content producers are saying "screw this" and leaving. The selfish upvoter in this case is being rewarded because "I got rid of the competition." Since we see this happening now, I'm sure we'll see it happen the other way around with downvotes, which is unfortunate.
Remember when Napster and Metallica made headlines? One side wanted entertainment for free and the artists didn't think it was fair to work for nothing. So here we are, years later. The artist/writer etc can get paid, and the content consumer can leave with more money than they had before they were entertained. If you would have told those who wanted entertainment to be free way back when that they could download their mp3 files AND get paid, they would have laughed at you and said you're crazy... but here we are.