The other day, I was also cheated out of $ 300 (but this was done by a person with whom I had exchanged several times before.)
Slightly late, but still: you should be more careful! I use exchanges whenever I can instead of person to person trading. As long as you don't keep funds on exchanges for a long time, I think it's safer.
I'm in a bad place right now, it's all about money.. I don't know what to do now, I hope everything will improve.
It's just something I've never encountered before.
It's a triangle scam, I've seen them before (but never experienced it myself):
A wants to buy something
B wants to sell something
C pretends to be A when talking to B, and B when talking to A. When A thinks he's paying B, he's actually paying C. Or, when B thinks he's sending the item to A, he's actually sending to C.
In the end, A and/or B get scammed, and C disappears.