JunkCoin https://bitcointalksearch.org/user/junkcoin-187416
Densitymax https://bitcointalksearch.org/user/densitymax-163537
Electronics4BTC https://bitcointalksearch.org/user/electronics4btc-197585
VirtualGoods https://bitcointalksearch.org/user/virtualgoods-318354 (this one hasn't scammed yet, but probably will in the future)
In all situations it appears that one or two accounts claim to have purchased what the user was selling then after that others try to buy from him, pay first and never receive the codes/merchandise.
Coming from the guy who is buying forum accounts to scam users. I am using escrow.