As this forum is monetized by ICO campaigns and services regarding bitcoin then you earn money for advertising on bitcointalk forum.
The Bitcointalk forum monetized by ads.
Selling accounts is highly risky because if noticed some accounts here are called trusted members and if they notice that there was an account trade done they will flag your account by simply giving you negative trust. With negative trust your account is limited to enter less campaigns. So there is huge risk involved if you are looking into buying one, not to mention that there are many scams around as well.
I don't think its has high risk to buy account instead it is prohibited, I'm actually against it since "Account sales encourage scams, spam, and account farming."