Is it allowed to sell signatures?
Yes, why not? In this thread you can find a list of ongoing signature campaigns
https://bitcointalk.org/index.php?topic=615953.0 There are probably some more which are not listed here, but the overview is quite good and useful.
Yes, signatures can help people promote their website or their online business. It also make people interesting and trusted. Even though in the process we do not know yet, how the business will go. But to answer your title questions, the answer is :
People sell their account because buyer want to have a account that have a high rank. The more high your account is, the more money you will get when you join a campaign in this forum. And this forum allow people to sell their account. But, some people say that buying account have consequences, because as they say : "instant way always have consequence"