That means you won't get banned if you're found selling or buying accounts. It doesn't mean you won't get negative trust.

Negative trust should be used when we think someone is untrustworthy.
Personally, I think it is if the sold account had positive trust or is trusted in some way, because the new owner can take advantage of that trust an scam easily.

I wouldn't leave negative trust when selling other kind of accounts, maybe a neutral but that's all.
However, there's no guidelines about this AFAIK, so everyone can have their own opinion about whether or not that's untrustworthy.