We have seen this happen too often.
First off, I am sorry this happened to you. You felt you could trust someone, and that person clearly violated that trust.
Second, share the name of the trader with everyone in the group. Do this for a couple reasons. We all need to know so this person doesn't do this to anyone in the group. More importantly, someone that knows this person will see this post, and call the guy up and tell him that he better pay up right away. Exposing the person is likely the cheapest and quickest way to get your money.
Written contracts hold up much better in court than a verbal agreement. We all feel awkward presenting a contract to someone on the hood of a pickup. I get that. However, if the other party has intentions of paying you they will gladly sign it. If they throw a fit about trust and whatnot, it is a great sign that you shouldn't be doing business with them.
Good luck.