I know account selling is prohibited in the forum
It isn't. It is just discouraged:
18. Having multiple accounts and account sales are allowed, but account sales are discouraged.
is it allowed to post telegram groups links in bitcointalk? AFAIK that's where all the black market deals are going on like merit trading, account selling etc...
Yes, it is allowed to post telegram group links because it is not telegram who encourages such actions, but it is the users who use it for negative things, and those negative things are negative only for us, since they are happening in plain chat windows which is what telegram provides to its users.
You cannot simply ban a platform not to be used just because a few people are using that platform to do evil stuff. It is not the thing that is good or bad, but it is the usage.
You cannot stop such actions from happening outside the forum as long as there are communicating medias available to them, and you cannot ban them all. Likewise, you cannot simply put a ban to having multiple accounts since there are people having multiple accounts for good reasons as well. There should be some other way to tackle this.