I don't think you can get a negative trust for buying an account. It rules says buying Bitcointalk account is "discouraged".
18. Having multiple accounts and account sales are allowed, but account sales are discouraged.
A neutral tag is likely to be given to a bought account with reference feedback to the change in password or posting style. IMO a negative feedback or red tag may be given in cases where the new owner is suspected to have malicious intent. Others had shared their opinion about this issue on this thread
Bitcointalk Rules..Bend/BreakAccount sales is fine if both parties agree to buy and pay without cheating or harming anyone, but in most cases selling an account will only increase cheater spammers. This is the reason why account sales are frowned upon even though they are allowed. I found some DT's gave them a red tag while others gave them a neutral tag, that's entirely up to them which I don't think is a bad thing.
I don't completely agree about the neutral tag for those account sales, it allows more users to be interested in buying or selling accounts so there will be more cases of scam and spammers on the forum. Someone should give them a red tag so that people will think that account sales is not frowned upon.