but why consider some rules says that "hacked/stolen" and that kind of things are not allowed, those topics are still there and not removed?
If there is sufficient evidence that indicates the items being sold are stolen or hacked the post will be removed. But like all free societies, everyone gets the benefit of the doubt.
why there is an option "locked/self-moderated" if someone will point you as "scammer" just because many of them doing that.
i mean, yeah i can see there is many scammers around, but not everyone is such, and how you can protect yourself from the competition if you do not moderate your own topic?
Form features aren't there to be abused by scammers, they have legitimate uses that have nothing to do with sales threads. No one will support the removal of valuable forum features just because scammers abuse them. And scammers will continue to scam even if those features aren't there.
and lets say someone had already experience in other platforms and it is familiar with the things around but not the things in this forum, why would some newbie rank will be called a scammer if he's not naturally stupid and can read how the things going around?
This isn't some other forum, this is Bitcointalk.org. Every forum I've ever visited has it's own culture. Take your time, get to know this one, and you won't aggravate the natives.
why should everyone be consider as scammer, if he use options provided by the forum no matter the rank ?
It has nothing to do with rank. There have been and likely still are plenty of high ranking members who are scammers. The specific threads you're talking about are commonly created by newbies because they're previous account was red-tagged. They create an account, get caught doing shady shit, get red-tagged, then create another account and start over. It's a never ending cycle.
lets say people come in this forum only for sales but not valuable as knowledge posts, they will never rank up and earn that "merit" to get high rank.
Lack of merit and rank don't prevent you from selling your goods. Use a trusted escrow, sell your goods only through the forum, don't sell stolen or hacked goods, and don't scam anyone. I guarantee you that none of the member who would be your "competition" are on DT, so their reviews would be essentially invisible. If any of them were to make allegations against you in your unlocked, non-self-moderated thread, they would need to show proof or no one will believe them. And, no highly reputed member is going to risk his reputation by making false allegations against a competitor.