I've seen a lot of people selling link building services and suddenly they got banned. I was just wondering if it was forbidden or not. Thanks!
You can't sell link building/profile creation/social bookmarking services but you can sell directory submission services.
Most link builders are serial spam ID creators, gratuitous promo linkdroppers, and start boatloads of "idiot threads"; - "What is SEO and how can it help my website? - "What does black hat mean?" - "{insert whatever along with the words Ugg Boots} Basically they tend to get banned because they do to a forum what what babies do to diapers. If someones primary income is derived from spamming forums and violating guidelines, allowing them to sell that product on your forum is just silly.