They do not, and neither does this site. Allowing those things to be posted here and kept here is part of preventing scams. It is preserving the evidence. If all scams were moderated and then deleted, then those users posting them could potentially could scam later because there is no evidence (thread with all of the evidence would have been deleted) remaining linking them to the scam. At that point, it would all just be the accusers' word against the scammer's. By keeping threads about those sites here, the evidence is kept, the OP is negged, and the evidence can be used in the future to link other accounts to the same scammer and prevent future scams.

AFAIK phishing sites and sites that have malware are removed.

What is wrong with selling used stuff? By no means are accounts being marketed as new, that is completely impossible. Since when has selling used goods been a bad thing?

The forum does not promote ponzis. In fact most ponzi operators are immediately negged, but as I said before, their threads are being kept as those threads can provide evidence. If you find a thread about a ponzi and you can't see the big red "Warning: trade with caution" under the poster's name and you are scammed and lose money, that is entirely your fault. The only reason the investor based games subforum exists is to keep all of those ponzis in the same location and so that all of them can get the same universal warning that those sites are gambling and/or scams.