Why? Post count or time registered doesn't make anyone trustworthy. If anyone wants to tell me otherwise, i'll tell them they are wrong. We have *extremely* strict rules when it comes to buying and selling. So we allow low-posters to buy and sell because anyone breaking the rules has their thread or post pulled, and warnings / bannings are quickly issued to repeat offenders (and our bannings arnt just from the trading section, its a forum permaban), usually within minutes, and those which dont get pulled quickly are avoided by the other members.
We're actually considered the safest to trade/sell on forum in our niche. With hundreds of trades happening every day, 99.9% of them all going on without issue.
In the last 18 months we have only had 1 legitimate issue, the others were all simply being impatient and panicing. Oh and that 1 legitimate issue, we have refunded the guy who got scammed the money from our site revenue, and are taking court action against the scammer.