There are a ton of these sellers on the marketplace forum, Wish a mod could sweep through there real quick and perm ban all of them, at least that will deter them a little bit.
They will not moderate such cases (unfortunately, there's no rule against it), so you have to report (create a thread) such users and let DT1/2 members, take the appropriate actions:
19. Possible (or real) scams and Trust ratings are not moderated (to prevent moderation abuse).
Q: Why haven't you banned <insert scammer username here> who is an obvious scammer?
A: Possible (or real, not for me to decide) scams are not moderated to prevent moderator abuse. If we start picking out which ones we call "scammers" and ban, we would make a lot of decisions based on biased opinions.
To OP:
7. Avoid buying/selling forum accounts.