The amount sold was over 500000 BTC - more that all estimated deposits on MtGox. The likely explanation is that these were just virtual BTCs added through SQL injection. The alternatives are that a single person was keeping that much on an exchange rumored to be insecure and used insecure password (easily cracked from leaked md5 hashes), or the person decided to sell half his BTC for $0.01 - an extra $2500 in addition to hundred thousands.