Link to scammer's Sythe profile: http://www.sythe.org/members/244784-madkilla.html

Briefly explain the trade: $3.50 (I was the buyer) for 1 Runescape Pin, valid for 1 month of membership.

How they scammed: He gave me an invalid pin on MSN, (when entered it said "Invalid Pin"), after which time he teamviewered me and entered the pin personally, and it was once again "Invalid". Then he disconnected from team viewer, and stopped responding to me on MSN.

Proof that you were not talking to an impostor: I received a PM from him during the trade, and the MSN I was speaking to was the same as in his profile.

PM:
Trade screenshots:
http://i46.tinypic.com/2z4k11t.jpg
http://i47.tinypic.com/2zi516r.jpg
http://i47.tinypic.com/2mz9m39.jpg
http://i45.tinypic.com/2ypbw4z.jpg

We can't really prove if a pin was already used or legitimately invalid. Use an MM in the future or record all pin trades.