First of all, thank you yahoo62278 for bringing this thread to my attention. I need to look more into the subject matter, have only skimmed the facts now.
Negative tag will be unjustified as the person did no harm, didn't fool anyone, didn't abuse any rule. While neutral tag will practically serve no purpose at all as the account is a pure bounty hunter. That thin-black numbers on his trust column will just serve to inform BMs that the account move hands, and most likely the campaign managers --if they're a wise manager enough-- couldn't bother less as long as if that person made a good posts and promote the campaign they're working on. So yeah, no purpose.
I don't think so. Tags are all about trust:

Tags are supposed to warn other users that a person is not very trustworthy and you should be very careful when trading coins with this person. The trading of coins has of course now shifted away from the forum and currently concerns rather collectibles and co., but the intention is the same: Would I enter into a trade with this person in good conscience or not.
So if someone has bought an account I would at least at the beginning under no circumstances enter into a trade and I think only a few of us would. So for me, in this situation, a negative tag would be quite justified. The tags can later be relatively easily converted into, for example, neutral tags when it has become clear how the new account owner behaves and whether he is basically trustworthy or not.