MtGox has internal transfers between accounts on their trading/accounting system. These are not actual transactions on the Bitcoin blockchain.

Read the FAQ. Transfers between MtGox accounts have never stopped working. https://www.bitcoinbuilder.com/faq.html

MtGox allows internal transfers.

GoxCoin Seller->Bitcoin Builder MtGox Account->GoxCoin Buyer

Bitcoin Buyer<-Bitcoin Builder Wallet<-Bitcoin Seller