When you see a locked scam topic, the best thing to do is click "report to moderator" and give a comment. Mods will take a look and if it is a scam, they will delete the topic and ban the member.
Mods do not delete scams and AFAIK they do not ban scammers either. The forum uses a caviet emptor policy (buyer beware).
The OP is correct in saying that a lot of people are creating threads selling something and then locking it so no one can question then in a public setting.