Hello,
I have seen a seller selling hacked accounts for really low on forum.
Can I know is it possible to hack that many accounts without knowing password or emails? Is there any loophole on forum?
The forum's database was leaked in 2015, the databasedump contained the hashed passwords of all users at that time.
Since the databasedump has been on sale on a couple of darknet marketplaces, some people started bruteforcing the user's passwords obtained from the database leak. So, if you have an account older than 2016, it was/is a good idear to change your password, cause sooner or later, one of the databasedump's buyers will probably be able to bruteforce your password.
Other sources might be successfull phising attempts and the fact that a lot of people re-use their passwords.
As for your last question: AFAIK, there is no current loophole in the forum. Only one or two people are able to access the database, that's about it.