It doesn't have to be an inherently bad thing.
Vote selling services can do a lot to prevent abuse, black- & whitelists could help in that matter.
But, as mentioned, I think that this is coming, nonetheless. It's a resource given to every account which has some kind of a value. Not really high, but it's something, so it will be tapped.
The wrong way would not to acknowledge this and not to start a discussion about it.
If someone buys a flag for me, I will flag 2.5 posts of theirs a day until the damage is paid back and more. I could also buy a flag for them and/or any associated account and it may be a lot worse than what they did to me capiche?
The person who is selling the flags will also get flagged. The loser could be permanently turned off of Steem and they aren't going to necessarily be poor content creators. As you can see whales and investors probably don't want this madness. All it takes is one whale to stomp any mercenaries profits away. If Tipu wants to be one of the least popular accounts on Steemit and watch its delegations dry up, it can go ahead and try to profit off of selling flags.
When I risk being flagged, I am going to stop posting and just lease out my Steem or sell it. Selling flags is as bad as selling slaps in the face, there had better be a damn good reason. Have some accountability, powerup and give your own flags! A blacklist will likely exist for anyone selling or buying flags.