Thanks for the warning. To be honest, I wouldn't have thought to ask a seller to open up a diff (or anything else) and provide photos of it. Though, I rarely consider buying something that doesn't have photos in the for sale post or at least an offer to send them by sms.
I've bought a few things off other members and haven't had cause for complaint yet. I have even put up the odd for sale and wanted post. The concern I have with a minimum number of posts before somebody can list an item for sale is that there is no way to ensure those posts contribute anything to the community. What is to stop somebody from jumping into a build thread and replying to every update with "Nice!"?
I don't have much of a post count, but I'd rather put up my extras for sale here where like-minded people could benefit from a lower price than if I flogged stuff out on gumtree. Then again, as I'm more of a buyer than a seller, it would hurt me a lot more to be locked out of seeing For Sale posts than it would hurt me to be locked out of making them.