Multiple accounts and and account selling should never be allowed on forums where business is conducted. It's fine on entertainment forums and places like reddit, but it's just a horrible idea to allow this on forums where you are dealing with buying and selling of currency and goods . This opens the door for scammers, shill bidding, and fake service reviews.
It can't be stopped sadly. Even if the moderators tried to stop it. They would deal outside the forum. For example reddit and other sites which has Bitcoin discussion. Most accounts which are sold are used to scamming and the things you have listed. But, some are also sold to allow people to get out of newbie jail. Although, that isn't very hard at all. So, I just consider the person who is selling them as a untrustworthy person because he/she is actually encouraging scamming.