From my point of view, he did a trade with someone and subsequently left positive trust for the person he traded with because everything went smoothly.
Let's say it's safe to asume that this deal is the sale of the account.
So you sell your alt account to someone and leave him feedback over this, to the account you owned yourself days before.
I am personally stingy in leaving positive trust, so this is not something I would do, especially considering that would likely be a deal in which I am receiving
BTC first.
I would find this similar to doing a currency exchange deal with a newbie, everything going smoothly and then leaving positive trust because of the smooth deal.
I would also say that it is not safe to assume that the trust is for the sale of the account because of the time difference. According to zazarb the deal was on Dec 17, however according to the screenshot in the thread in meta(that will likely get moved to reputation), the trust was left ~2 weeks later on the 29th
edit:
you edited your post to include
However, a neutral should sufficient.
This may or may not be true. If you feel this way then you should not take his trust ratings into consideration. Leaving trust for a trade that did not take place is something that I would consider to be scammy. Leaving trust for a trade that I feel that trust should not have been left for is something that would cause me to exclude them from my trust network