Trust shouldn't be removed just because an account is sold. If this was regular practice people would just fake a sale to themselves in an attempt to get it removed. Accounts should be ruined or lose most of their value once they have used to scam and if people removed feedback under these circumstances scammers could get a lot more for their accounts or as above cheat the feedback into being removed completely.
IMO if the buyer provides certain credible proof that he's not associated with the seller, then maybe the trust should be changed to neutral, but still providing the same information.
The problem is that the buyer has 0% chance of proving that he's not associated with the seller at all.
How can you prove you're not associated with someone? That can easily be faked anyway and would be abused in the way I detailed above. Scammers would then easily be able to get their accounts restored to normal. If you buy a negatively trusted account then you have bought a negatively trusted account.