Becoming a member has more to do with being interactive and participate on the rest of the forums, making friends, asking questions regarding reef related topics, (even so you see a lot members selling stuff on that forum, it becomes more trusted when you know that person and that the stuff you buying has no issues at all, buying from a well know and trusted member makes you feel more comfortable I believe)
After all, being member of the community brings more benefit instead of just the sole proposit of selling, I myself been trying to sell what I don't need anymore and at the same time I have bought great equipment that I wouldn't find in another place, at the same time I have met helpful people. Hope it helps in your desicion...