I am surprised that others do not know the reason for purchasing an account, most likely joining signature campaigns, as many campaigns are now accepting Full-Memeber rank, with about 120 posts (at least) and about 4 months of activity, that period is not enough to track the writing style  and therefore It's easy to buy an account without being noticed.

The solution to this problem is that Sr. Member rank is the lowest rank to join signature campaigns, it will be very difficult for scammers/account sellers.