It only shows, that not each account seller/buyer seems to be a scammer. So give all the same attention and chances, before tagging down or tag them all, what would be also ok.
I think there needs to be a deterrent. You seem to overlook the utility of leaving negs for account sellers--true, there's no scientific study showing that such an effect exists, but in my eyes it beats doing absolutely nothing on a forum where people are allowed to scam (and do), people are allowed to shitpost at will (and do), and where we're told to give the benefit of the doubt where users buy, sell and trade merits, even when it's obvious what they're doing.
I just gave a neg to a guy who is obviously an account farmer for selling accounts of all shapes and sizes:
https://bitcointalk.org/index.php?topic=3089343.0I feel very warm and fuzzy doing this, though he probably won't care and apparently thinks that any negs left for him are from his competitors. This guy is facilitating scams, shitposting, and account farming, and is letting others gain access to rank (and perhaps trust, though this isn't a selling point here) that the buyer did not earn. Fuck 'em.