The question has been asked before but I'm sorry to say I can't really tell you right off. Not that I don't want to, I just can't remember.
Seems like we initially set it at 10 or 15 or something. There's no rule as to what the posts must be, but we don't like people to simply spam to reach a certain number (Sheep is the exception!
).
The reason behind it was, as has been mentioned, we were getting spammers selling Viagra knockoffs and cloned cell phones. It's a lot easier if we weed those types out by requiring a minimum post count. As a side effect people are usually more confident buying from or selling to someone they "know" a little bit. Anyone
could rip you off, but a long time forum member with a lot of posts usually won't risk their reputation here by screwing you.