I think there are a couple of points here, we can all judge those who race to 50 posts and suddenly put something up for sale, and then those who are here to learn and have a bit of banter. Yes it does all come down to trust at the end of the day, but this works both ways from both the seller and as we have recently seen the buyer as well.
I've made quite a number of purchases from on here, in fact all my kit has been purchased from either forum members or sponsors! I had no problems purchasing of forum members all who had different levels of posts, I certainly wouldn't buy off anyone who had been on here a matter of weeks and raced to 50 posts, but I do look over someone's history and try and get a feel for them before making my decision.
In general I think that the forum is well run and that the Shaq's/Brimble's are in a minority compared to the most of us who suddenly realise we have more kit than we could possibly ever need!