I wouldn’t call the “red tag for switching hands” much of a reason considering any user can give positive or negative reputation. However, I would call the “This user's email address was changed recently” pop up suspicious combined with the other evidence.
Try to explain to me the reason for my actions in changing my password and email. I just foresee the scope of your imagination, and maybe one will make sense. But you would be very mistaken to think that the account changed ownership due to a password change. There can be many reasons.
But as for the negative tag left by a DT member, this is a pretty serious comment, and you probably don’t know the meaning of these ratings if you say that anyone can leave such a tag. The difference between any person and a person from DT is big.
https://bitcointalk.org/index.php?topic=5191802.0Normally I don't see the act of changing your email suspicious at all but combined with the accusations here it does add up as some form of evidence, although not exactly a strong form of evidence, still noteworthy to point out. Especially seeing that his password hasn't changed either or else the pop up would've shown. Even then someone could just be like "he used a personal password specifically for bitcointalk and gave it to the buyer".
As for the person who gave him the reputation being a DT Member, that I was unaware of. I read the thread you linked and it was actually pretty helpful since I finally got to see what "DT" was after seeing it used literally everywhere on the forum. I knew they were like some sort of group of trusted people who were highly respected in the community that expose scammers and those type of stuff. But I never knew what the term "DT" meant or what else they did. I thought the person giving out the reputation was a normal user up until you pointed this out. This is because I haven't used this forum actively enough to know everyone yet. Since he is a DT member then he definitely must've reviewed the evidence and thoroughly investigated the situation out. So in the end, maybe it is a good point to keep in afterall along with his other points.