The people who are legit for a long time and then turn crooked are the hardest to avoid. You can do your due diligence and deal with them based off their track record, but it ends up being meaningless.
We had a guy in a car club I've been a member of for years. He was club president for a period, attended meets with his family, and sold many parts to people. At some point he ran into financial problems and started selling off a lot of stuff. Many club members stepped up and paid on the upper end for stuff to help him out, and he ended up scamming all of them and never actually delivered anything. He admitted it openly, and said he was just doing what he had to do to put food on the table. I understand being in a bad situation, but stealing from someone (especially a friend) is never OK. Had he been honest I know a bunch of us would have thrown some money his way to help out. That whole situation really shook me up and made me a lot more cynical.