Kuanaco it's possible that you are spamming to get people to visit that website.
Just in case yours is a genuine question, I give you the benefit of the doubt.
The answer depends whether you want your members to discuss useful things with each other. That is what forums do well.
I don't really see how buyers of domains need to talk to each other.
You can set a forum so visitors can message the admin staff, but not see each others; posts. You might find that useful as a support/ FAQ service. But there are site chat software to do that as a specialisy thing without all the extra features of a forum.
A forum is aimed to encourage contact between users, whether members only, or members + guests.
Forum software is meant as a solution for online discussion or support, not articles / classifieds although there are add-ons that build on forum software that allow for such an expansion of a forum community.
Well I have one site where I basically use XF as a static site editor - for article pages.
No members just me doing the articles. Sidebar on all pages and one forum I use for a kind of FAQ, one post per thread.. A slightly custom contact form with a few extra fields.
It's worth it for me to
a) have an easy frontend editor w/o html pages
b) XF designing is so nice
c) backend admin is sleek and familiar
c) I don't have to cope with a separate script admin for the contact page
d) this site has a partner site which IS an active forum so they match in 'look.'
But that's a specialist set of needs and maybe a bit quirky.